Methods for Error Correction with Resistive Change Element Arrays

ABSTRACT

Error correction methods for arrays of resistive change elements are disclosed. An array of resistive change elements is organized into a plurality of subsections. Each subsection includes at least one flag bit and a plurality of data bits. At the start of a write operation, all bits in a subsection are initialized. If any data bits fail to initialize, the pattern of errors is compared to the input data pattern. The flag cells are then activated to indicate the appropriate encoding pattern to apply to the input data to match the errors. The input data is then encoded according to this encoding pattern before being written to the array. A second error correction algorithm can be used to correct remaining errors. During a read operation, the encoding pattern indicated by the flag bits is used to decode the read data and retrieve the original input data.

TECHNICAL FIELD

The present disclosure relates generally to error correction methodssuitable for use with resistive change element arrays, and, morespecifically, to such error correction methods that reduce parityoverhead and improve error decoding latency within resistive changeelement arrays while still maintaining a required correctable bit errorrate (BER).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following U.S. patents, which areassigned to the assignee of the present application, and are herebyincorporated by reference in their entirety:

-   -   U.S. Pat. No. 6,835,591, filed on Apr. 23, 2002, entitled        Methods of Nanotube Films and Articles;    -   U.S. Pat. No. 7,335,395, filed on Jan. 13, 2003, entitled        Methods of Using Pre Formed Nanotubes to Make Carbon Nanotube        Films, Layers, Fabrics, Ribbons, Elements, and Articles;    -   U.S. Pat. No. 6,706,402, filed on Mar. 16, 2004, entitled        Nanotube Films and Articles;    -   U.S. Pat. No. 7,115,901, filed on Jun. 9, 2004, entitled        Non-Volatile Electromechanical Field Effect Devices and Circuits        Using Same and Methods of Forming Same;    -   U.S. Pat. No. 7,365,632, filed on Sep. 20, 2005, entitled        Resistive Elements Using Carbon Nanotubes;    -   U.S. Pat. No. 7,781,862, filed on Nov. 15, 2005, entitled        Two-Terminal Nanotube Devices and Systems and Methods of Making        Same;    -   U.S. Pat. No. 8,513,768, filed on Aug. 8, 2007, entitled        Nonvolatile Nanotube Diodes and Nonvolatile Nanotube Blocks and        Systems Using Same and Methods of Making Same;    -   U.S. Pat. No. 8,541,843, filed on Aug. 6, 2009, entitled        Nonvolatile Nanotube Programmable Logic Devices and a        Nonvolatile Nanotube Field Programmable Gate Array Using Same;    -   U.S. Pat. No. 8,000,127, filed on Nov. 13, 2009, entitled Method        for Resetting a Resistive Change Memory Element;    -   U.S. Pat. No. 8,619,450, filed on Sep. 1, 2010, entitled A        Method for Adjusting a Resistive Change Element Using a        Reference;    -   U.S. Pat. No. 9,263,126, filed on Aug. 12, 2014, entitled Method        for Dynamically Accessing and Programming Resistive Change        Element Arrays; and    -   U.S. Pat. No. 9,299,430, filed on Jan. 22, 2015, entitled        Methods for Reading and Programming 1-R Resistive Change Element        Arrays.

This application is related to the following U.S. patent applications,which are assigned to the assignee of the application, and are herebyincorporated by reference in their entirety:

-   -   U.S. patent application Ser. No. 14/812,173, filed on Jul. 29,        2015, entitled DDR Compatible Memory Circuit Architecture for        Resistive Change Element Arrays; and    -   U.S. patent application Ser. No. 15/136,414, filed on Apr. 22,        2016, entitled Methods for Enhanced State Retention within a        Resistive Change Cell.

BACKGROUND OF THE INVENTION

Any discussion of the related art throughout this specification shouldin no way be considered as an admission that such art is widely known orforms part of the common general knowledge in the field.

Resistive change devices and arrays, often referred to as resistanceRAMs by those skilled in the art, are well known in the semiconductorand electronics industry. Such devices and arrays, for example, include,but are not limited to, phase change memory, solid electrolyte memory,metal oxide resistance memory, and carbon nanotube memory such as NRAM™.

Resistive change devices and arrays store information by adjusting aresistive change element, typically comprising some material that can beadjusted between a number of non-volatile resistive states in responseto some applied stimuli, within each individual array cell between twoor more resistive states. For example, each resistive state within aresistive change element cell can correspond to a data value which canbe programmed and read back by supporting circuitry within the device orarray.

For example, a resistive change element might be arranged to switchbetween two resistive states: a high resistive state (which mightcorrespond to a logic “0”) and a low resistive state (which mightcorrespond to a logic “1”). In this way, a resistive change element canbe used to store one binary digit (bit) of data.

Or, as another example, a resistive change element might be arranged toswitch between four resistive states, so as to store two bits of data.Or a resistive change element might be arranged to switch between eightresistive states, so as to store three bits of data. Or a resistivechange element might be arranged to switch between 2″ resistive states,so as to store n bits of data.

In some cases, a resistive change element may exhibit a higher errorrate when attempting to place it into one of its resistive statescompared to other resistive states. This error bias can be substantiallylarge. For example, two-state resistive change element with states SETand RESET may show an error distribution of 80% RESET and 20% SET,wherein RESET errors occur four times as often as SET errors. In suchcircumstances, it is possible to design an error correction method whichtakes advantage of this error bias to significantly reduce the relianceon traditional error correction methods.

As arrays of resistive change elements are increasingly used to createflash memories, solid state drives (SSDs), and the like in the currentstate of the art, there is a growing need for improved error detectionand correction algorithms specifically designed for resistive changememory arrays to reduce parity overhead and latency. To this end, thepresent disclosure provides such improved error correction methods.

SUMMARY OF THE INVENTION

The present disclosure relates to error correction methods for arrays ofresistive change elements and, more specifically, to such errorcorrection methods that provide reduced parity overhead and latencywhile still maintaining a required correctable bit error rate (BER).

In particular, the present disclosure provides a method for errorcorrection within a resistive change element array. The method comprisesfirst dividing the array into a plurality of subsections, eachsubsection comprising one flag cell and a plurality of data cells. Eachof the data cells is comprised of a resistive change element capable ofbeing adjusted between two non-volatile resistive states. The methodnext comprises receiving a set of input data to be programmed into thearray. The method then comprises initializing all of the data cellswithin each subsection to an initial condition and all of the flag cellsto a deactivated state. The method then comprises activating the flagcell within any of the subsections in which a data cell failed toinitialize and is due to be programmed into a state matching the initialcondition according to the set of input data.

The method next comprises programming each subsection with aninactivated flag cell according to the input data and each subsectionwith an activated flag cell according to the logical inverse of theinput data to realize a set of programmed data for each subsection. Themethod next comprises accessing the programmed data during a readoperation to realize a set of output data for each subsection. Finally,the method comprises inverting said output data for each subsection withan activated flag cell, wherein the step of inverting according to theflag cells provides a first error correction operation.

According to one aspect of the present disclosure the method furthercomprises encoding the input data according to a second error correctionoperation prior to the programming step and decoding the output dataaccording to this second error correction operation subsequent to theaccessing step.

According to another aspect of the present disclosure the second errorcorrection operation is Bose-Chaudhuri-Hocquenghem (BCH) errorcorrection algorithm.

According another aspect of the present disclosure the combined parityoverhead from the first error correction operation and the second errorcorrection operation is less than the parity overhead of the seconderror correction operation used alone for an equivalent correctable biterror rate (BER).

According another aspect of the present disclosure the combined decodinglatency for the first error correction operation and the second errorcorrection operation is less than the decoding latency of the seconderror correction operation used alone for an equivalent correctable biterror rate (BER).

According another aspect of the present disclosure the first errorcorrection operation corrects initialization errors and the second errorcorrection operation corrects programming errors.

According another aspect of the present disclosure the total number oferrors does not change no matter which error correction scheme is used.The initialization error will be found as programming error by usingconventional error correction scheme, BCH along.

According to another aspect of the present disclosure the initialcondition for a resistive change element array is a RESET state and thefirst error correction operation is capable of correcting RESET errors.

According to another aspect of the present disclosure the initialcondition for a resistive change element array is a SET state and thefirst error correction operation is capable of correcting SET errors.

According to another aspect of the present disclosure the first errorcorrection operation is capable of correcting at least oneinitialization error per subsection within the resistive change elementarray.

According to another aspect of the present disclosure the first errorcorrection operation and the second error correction operation provide apreselected correctable bit error rate (BER).

According to another aspect of the present disclosure the preselectedbit error correction rate is on the order of 0.1%.

According to another aspect of the present disclosure the preselectedbit error correction rate is on the order of 0.2%.

According to another aspect of the present disclosure the resistivechange elements are two-terminal nanotube switching elements comprisinga nanotube fabric.

According to another aspect of the present disclosure the resistivechange elements are metal oxide memory elements.

According to another aspect of the present disclosure the resistivechange elements are phase change memory elements.

According to another aspect of the present disclosure the resistivechange element array is a memory array.

Other features and advantages of the present disclosure will becomeapparent from the following description of the invention, which isprovided below in relation to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an exemplary layout of a vertically orientedresistive change cell.

FIG. 1B illustrates an exemplary layout of a horizontally orientedresistive change cell.

FIG. 2 is a simplified schematic illustrating an exemplary architecturefor an array of resistive change elements wherein FET selection devicesare used within the cells of the array.

FIG. 3 is table detailing READ and programming voltages required foradjusting or inspecting CELL00 of the array architecture illustrated inFIG. 2.

FIG. 4 is a simplified schematic illustrating an exemplary architecturefor an array of resistive change elements wherein no selection devicesor other current limiting circuitry are used within the cells of thearray.

FIG. 5 is a perspective drawing illustrating the layout of a 3D array of1-R resistive change element cells

FIG. 6 is a graph plotting parity overhead for memory arrays usingconventional Bose-Chaudhuri-Hocquenghem (BCH) error correctionalgorithms.

FIG. 7A is a graph plotting the error rates observed from an exemplaryresistive change element array (both SET and RESET errors).

FIG. 7B is a graph plotting breaking out only the RESET errors from theerror rates observed from an exemplary resistive change element array(as shown in FIG. 7A).

FIG. 7C is a graph plotting breaking out only the SET errors from theerror rates observed from an exemplary resistive change element array(as shown in FIG. 7A).

FIG. 8A is a diagram of an exemplary resistive element array with8-kilobyte user data divided into eight blocks with 1-kilobyte userdata, each block using conventional BCH error correction alone.

FIG. 8B is a diagram showing a block of FIG. 8A in more detail.

FIG. 8C is a table detailing the parity overhead for differentcorrectable bit error rate (BER) for the exemplary 8-kilobyte resistiveelement array of FIG. 8A (using conventional BCH error correctionalone).

FIG. 9A is a diagram of an exemplary resistive element array with8-kilobyte user data divided into eight blocks with 1-kilobyte userdata, each block divided into four array subsections and using thereverse flag error correction (RFEC) method of the present disclosure.

FIG. 9B is a diagram showing a 1-kilobyte block of FIG. 9A in moredetail.

FIG. 9C is a table detailing the parity overhead for differentcorrectable bit error rate (BER) for the exemplary resistive elementarray of FIG. 9A (using the RFEC method of the present disclosure).

FIG. 10 is a flow chart detailing an exemplary programming/read backoperation that uses the reverse flag error correction (RFEC) method ofthe present disclosure.

FIG. 11A is a diagram illustrating an exemplary error correctionoperation according to the RFEC method of the present disclosure used tocorrect a single RESET error on a cell intended to be written with alogic “0”.

FIG. 11B is a diagram illustrating an exemplary error correctionoperation according to the RFEC method of the present disclosure used tocorrect a single RESET error on a cell intended to be written with alogic “1”.

FIG. 11C is a diagram illustrating an exemplary error correctionoperation according to the RFEC method of the present disclosure used tocorrect a single SET error.

FIG. 11D is a diagram illustrating an exemplary error correctionoperation according to the RFEC method of the present disclosure used tocorrect two RESET errors on two cells intended to be written with alogic “0”.

FIG. 11E is a diagram illustrating an exemplary error correctionoperation according to the RFEC method of the present disclosure used tocorrect two RESET errors on two cells wherein one is intended to bewritten with a logic “0” and the other is intended to be written with alogic “1”.

FIG. 12A is a simplified schematic illustrating an exemplary encodingcircuit for use with the RFEC method of the present disclosure.

FIG. 12B is a simplified schematic illustrating an exemplary decodingcircuit for use with the RFEC method of the present disclosure.

FIG. 13A is a table detailing parity overhead for different subsectionsize configurations on an exemplary array with 8-kilobyte data using thereverse flag error correction (RFEC) method of the present disclosure toobtain a correctable bit error rate (BER) of 0.2%.

FIG. 13B is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIG. 13A (using the RFEC method of thepresent disclosure) with the parity overhead required for conventionalBCH methods alone.

FIG. 13C is a table detailing parity overhead for different subsectionsize configurations on an exemplary array with 8-kilobyte data using theRFEC method of the present disclosure to obtain a correctable bit errorrate (BER) of 0.3%.

FIG. 13D is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIG. 13C (using the RFEC method of thepresent disclosure) with the parity overhead required for conventionalBCH methods alone.

FIG. 14A is a table detailing parity overhead for different subsectionsize configurations on an exemplary array with 8-kilobyte data using thedual reverse flag error correction (DRFEC) method of the presentdisclosure to obtain a correctable bit error rate (BER) of 0.2%.

FIG. 14B is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIG. 14A (using the DRFEC method of thepresent disclosure) with the parity overhead required for conventionalBCH methods alone.

FIG. 14C is a table detailing parity overhead for different subsectionsize configurations on an exemplary array with 8-kilobyte data using theDRFEC method of the present disclosure to obtain a correctable bit errorrate (BER) of 0.3%.

FIG. 14D is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIG. 14C (using the DRFEC method of thepresent disclosure) with the parity overhead required for conventionalBCH methods alone.

FIG. 15A is a table detailing parity overhead for different subsectionsize configurations on an exemplary array with 8-kilobyte data using theadvanced bit flip (ABF) method of the present disclosure to obtain acorrectable bit error rate (BER) of 0.2%.

FIG. 15B is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIG. 15A (using the ABF method of the presentdisclosure) with the parity overhead required for conventional BCHmethods alone.

FIG. 15C is a table detailing parity overhead for different subsectionsize configurations on an exemplary array with 8-kilobyte data using theABF method of the present disclosure to obtain a correctable bit errorrate (BER) of 0.3%.

FIG. 15D is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIG. 15C (using the ABF method of the presentdisclosure) with the parity overhead required for conventional BCHmethods alone.

FIG. 16A is a bar graph comparing the parity overhead for an exemplarythirty-two subsection array configured for use with the RFEC, DRFEC, andABF methods of the present disclosure with the parity overhead requiredfor conventional BCH methods alone at five correctable bit error rates.

FIG. 16B is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIGS. 13A, 14A, and 15A (using the methods ofthe present disclosure) with the parity overhead required forconventional BCH methods alone to achieve a correctable BER of 0.2%.

FIG. 16C is a bar graph comparing the parity overhead of the arrayconfigurations detailed in FIGS. 13A, 14A, and 15A (using the methods ofthe present disclosure) with the parity overhead required forconventional BCH methods alone to achieve a correctable BER of 0.3%.

FIG. 17A is a diagram of an exemplary resistive element array with8-kilobyte user data divided into eight blocks with 1-kilobyte userdata, each block divided into four array subsections and using the dualreverse flag error correction (DRFEC) method of the present disclosure.

FIG. 17B is a diagram showing a 1-kilobyte block of FIG. 17A in moredetail.

FIG. 18 is a flow chart detailing an exemplary programming/read backoperation that uses the DRFEC method of the present disclosure.

FIG. 19A is a diagram illustrating an exemplary error correctionoperation according to the DRFEC method of the present disclosure usedto correct a single RESET error on an odd cell intended to be writtenwith a logic “0”.

FIG. 19B is a diagram illustrating an exemplary error correctionoperation according to the DRFEC method of the present disclosure usedto correct a single RESET error on an even cell intended to be writtenwith a logic “0”.

FIG. 19C is a diagram illustrating an exemplary error correctionoperation according to the DRFEC method of the present disclosure usedto correct a single SET error.

FIG. 19D is a diagram illustrating an exemplary error correctionoperation according to the DRFEC method of the present disclosure usedto correct two RESET errors on one even and one odd cell intended to bewritten with a logic “0”.

FIG. 19E is a diagram illustrating an exemplary error correctionoperation according to the DRFEC method of the present disclosure usedto correct two RESET errors on one even and one odd cell wherein one isintended to be written with a logic “0” and the other is intended to bewritten with a logic “1”.

FIG. 19F is a diagram illustrating an exemplary error correctionoperation according to the DRFEC method of the present disclosure usedto correct two RESET errors on two odd cells wherein one is intended tobe written with a logic “0” and the other is intended to be written witha logic “1”.

FIG. 20A is a simplified schematic illustrating an exemplary encodingcircuit for use with the DFREC method of the present disclosure.

FIG. 20B is a simplified schematic illustrating an exemplary decodingcircuit for use with the DFREC method of the present disclosure.

FIG. 21A is a diagram showing eight exemplary data encoding patterns andcorresponding pattern reference codes (PRCs) for use with the three-flag2-bit advanced bit flip (ABF) method of the present disclosure.

FIG. 21B is a diagram showing all 24 possible encoding patterns for a2-bit subset for use with the 2-bit ABF methods of the presentdisclosure.

FIG. 22A is a diagram of an exemplary resistive element array with8-kilobyte user data divided into eight blocks with 1-kilobyte userdata, each block divided into four array subsections and using thethree-flag advanced bit flip (ABF) method of the present disclosure.

FIG. 22B is a diagram showing a 1-kilobyte block of FIG. 22A in moredetail.

FIG. 23 is a flow chart detailing an exemplary programming/read backoperation that uses the ABF methods of the present disclosure.

FIG. 24A is a diagram illustrating an exemplary error correctionoperation according to the three-flag 2-bit ABF method of the presentdisclosure used to correct two RESET errors on two even cells whereinone is intended to be written with a logic “0” and the other is intendedto be written with a logic “1”.

FIG. 24B is a diagram illustrating an exemplary error correctionoperation according to the three-flag 2-bit ABF method of the presentdisclosure used to correct two RESET errors on two odd cells intended tobe written with a logic “0”.

FIG. 24C is a diagram illustrating an exemplary error correctionoperation according to the three-flag 2-bit ABF method of the presentdisclosure used to correct a single SET error.

FIG. 24D is a diagram illustrating an exemplary error correctionoperation according to the three-flag 2-bit ABF method of the presentdisclosure used to correct two RESET errors on one even and one odd cellwherein the even cell is intended to be written with a logic “0” and theodd cell is intended to be written with a logic “1”.

FIG. 24E is a diagram illustrating an exemplary error correctionoperation according to the three-flag 2-bit ABF method of the presentdisclosure used to correct two RESET errors on one even and one odd cellintended to be written with a logic “0”.

FIG. 24F is a diagram illustrating an exemplary error correctionoperation according to the three-flag 2-bit ABF method of the presentdisclosure used to correct three RESET errors on one even and two oddcells wherein one odd cell is intended to be written with a logic “0”and the others are intended to be written with a logic “1”.

FIG. 25A is a scatter plot showing the parity overhead necessary toachieve five correctable BERs for various subsection sizes using thefive-flag 2-bit ABF method.

FIG. 25B is a table comparing the parity overhead necessary to achieve a0.2% correctable BER for each of the DFEC methods of the presentdisclosure.

FIG. 25C is a table showing the optimal subsection configuration and thecorresponding parity overhead for each of the DFEC methods of thepresent disclosure for five correctable BERs.

FIG. 26A is a diagram illustrating an exemplary error correctionoperation according to the five-flag 2-bit ABF method of the presentdisclosure used to correct at least two initialization errors in asubsection of a resistive change element array.

FIG. 26B is a diagram illustrating an exemplary error correctionoperation according to the five-flag 2-bit ABF method of the presentdisclosure used to correct a maximum of 100% of initialization errors ina subsection of a resistive change element array due to the favorableerror pattern.

FIG. 26C is a diagram illustrating an exemplary error correctionoperation according to the five-flag 2-bit ABF method of the presentdisclosure used to correct a minimum of 50% of initialization errors ina subsection of a resistive change element array due to the unfavorableerror pattern.

FIG. 26D is a diagram illustrating an exemplary error correctionoperation according to the five-flag 2-bit ABF method of the presentdisclosure used to correct between the minimum and maximum percentage ofinitialization errors in a subsection of a resistive change elementarray due to the error pattern.

FIG. 26E is a diagram illustrating an exemplary error correctionoperation according to the five-flag 2-bit ABF method of the presentdisclosure used to identify the optimal encoding pattern for anexemplary error pattern in a resistive change element array.

FIG. 26F is a diagram illustrating an exemplary error correctionoperation according to the five-flag 2-bit ABF method of the presentdisclosure used to identify the optimal encoding pattern for anexemplary error pattern in a resistive change element array with anadditional matching step.

FIG. 27 is a simplified block diagram of a resistive change elementarray memory system suitable for use with the error correction methodsof the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to error correction methods for resistivechange element arrays. More specifically, the present disclosure teachesdistributed flag error correction (DFEC) methods for resistive changeelement arrays that provide reduced parity overhead and reduced latencyas compared with traditional error correction methods. Within certainapplications the error correction methods of the present disclosure areused in concert with conventional error correction techniques to reducethe number of parity bits required for a desired correctable bit errorrate (BER).

According to the error correction methods of the present disclosure, ablock of resistive change elements within an array is divided into anumber of subsections by distributing flag cells across the memoryblock. The parity overhead added by the distributed flags may becalculated by dividing the number of flag cells by the number of datacells remaining in the subsection (e.g., 1/512 or 0.2% for a 512-bitsubsection with one flag bit). At the start of a write operation, theentire subsection is put into an initial state (either a SET or a RESETstate, as befits the needs of a specific application). Each subsectionis then checked to determine if any cells failed to initialize into therequired state. If any errors are found within the subsection, the errorpattern (that is, the number and position of errors in the subsection)is then compared to the input data (that is, the data to be written tothe subsection) in a data pattern matching (DPM) step. If the input datafor an error cell is the opposite of the selected initialization state,then the error cell is already in the desired programming state andthere is effectively no error. If the input data does not match thecurrent logical state of the error cell, however, one or more of thesubsection flags are activated to indicate the appropriate encodingpattern to apply to the input data in order to correct the error(s)according to a pattern reference code (PRC).

The input data for the subsection is then encoded according to theencoding pattern indicated by the pattern reference code (PRC) stored inthe flag bits. If the pattern reference code (PRC) indicates there areno effective errors, the input data may remain unchanged. Otherwise theinput data is selectively inverted to match the error pattern of thesubsection such that the error cells are now in the desired programmingstate and are no longer effectively an error. Next, the DFEC encodedinput data, along with the flag bits activated with the correspondingpattern reference code (PRC), is processed with a conventional errorcorrection method (such as, but not limited to, aBose-Chaudhuri-Hocquenghem (BCH) error correction algorithm) and thenprogrammed into the subsection. Since the entire subsection has alreadybeen initialized to either a SET or a RESET state (aside from any errorcells), only those data cells that require values opposite theinitialized state (that is, a logic “1” if the subsection wasinitialized into a RESET state, or a logic “0” if the subsection wasinitialized into a SET state) need to be programmed.

After the input data is written, it can be read out and decoded usingthe conventional error correction method to correct any remainingerrors, including write errors, data retention errors, and anyinitialization errors which were not corrected by the DFEC methods. If aflag cell is in an activated state, however, the read data may differfrom the input data as it has been further encoded according to a DFECencoding pattern. A reverse data pattern matching (RDPM) step is thenperformed, wherein the encoding pattern indicated by PRC stored in theflag bits is reversed to decode the read data and thereby recover theoriginal input data. In this way, the initialization and programmingerrors in a memory array may be corrected by combining the DFEC methodswith a secondary error correction method.

The present disclosure provides three examples of DFEC methods usingdifferent numbers of flag bits per subsection which are distributedacross a memory block. The number of flag bits used to delimitate asubsection determines the number of error patterns which may be matchedto the input data in a data pattern matching (DPM) step. Thus, a singleflag may match the input data according to two patterns (inverted andnot inverted), two flag bits may match the input data according to fourpatterns, three flag bits may match the input data according to eightpatterns, and N flag bits may match the input data according to 2^(N)patterns. Increasing the number of distributed flag bits may allow thecorrection of a larger number of errors by employing more complexpatterns, thereby reducing the number of parity bits required for asecond error correction algorithm while also increasing the DFECoverhead. Further, each method may be ideally suited to correct certaintypes and patterns of errors, and may introduce differing amounts oflatency. The DFEC method employed, and the specific configurationthereof, may therefore be chosen according to the requirements of aspecific application, as will be discussed in depth below.

In a first DFEC method, referred to as RFEC (reverse flag errorcorrection) or single-flag DFEC, a single flag bit per subsection isused to indicate that at least one data cell in the subsection is in anincorrect state following initialization (that is, for instance, a cellin a logic “1” state which is due to be programmed to a logic “0”). Aswill be described in more detail with respect to FIGS. 9A-C, 10, and11A-E, if at least one effective initialization error is detected duringa data pattern matching (DPM) step following initialization of thesubsection, the flag bit is activated in order to invert the input databefore it is written to the array. Thus, for example, an error cellwhich was originally intended to be written as a logic “0” would now bewritten as a logic “1”, and the error is effectively corrected. Once thesubsection is read, a reverse data pattern matching (RDPM) step detectswhether the flag bit is activated. If so, the read data is invertedagain to recover the input data.

Similarly, a second DFEC method uses two flag bits per subsection,referred to as dual reverse flag error correction (DRFEC) or dual-flagDFEC. In this case, one flag bit indicates an effective initializationerror in the even bits within the subsection while the other indicatesan effective initialization error in the odd bits. As will be describedin more detail with respect to FIGS. 16A-C, 17, and 18A-E, followinginitialization of the subsection, a DPM step activates one or both ofthe flags if an effective initialization error is detected in theirrespective data cells. Thus, if an effective initialization error isdetected in at least one even data cell the even flag bit is activated,and if at least one initialization error is detected in an odd data cellthe odd flag bit is activated. Following this DPM step, the input datais encoded by inverting all of the bits corresponding to the activatedflags. Thus, if the even flag bit is activated all of the even bits inthe input data are inverted, and if the odd flag bit is activated all ofthe odd bits in the input data are inverted. The encoded input data isthen written to the array. Following a read operation, a RDPM step isperformed to detect the PRC indicated by the flag bits. The bits in theread data corresponding to an activated flag are then inverted again torecover the input data.

A third DFEC method, referred to as advanced bit flip (ABF) DFEC, usesthe flag bits in each subsection to store a pattern reference code (PRC)which refers to an encoding pattern. In this method, all of the databits within the array are divided into data subsets (that is, groups oftwo or more data bits grouped together) during the data pattern matching(DPM) and reverse DPM (RDPM) steps. If one or multiple effectiveinitialization errors are detected within the subsection, a DPM step isinitiated which matches the input data to the array subsets containingat least one error to select an encoding pattern from a set ofreversible encoding patterns. Each encoding pattern provides a way totranslate any given logical pattern of bits within each data subset (10,01, 00, 11, for instance) into an encoded pattern (11 is encoded as 00,or 10 is encoded as 11, for example), so as to correct initializationerrors in the subsection. The pattern reference code (PRC) correspondingto the matched encoding pattern is then stored in the flag bits, and thedata subsets within the input data are encoded using the matchedencoding pattern before being written to the subsection. In order torecover the input data, a RDPM step is initiated during the readoperation in which the encoding pattern is reversed to recover the inputdata. In this way, the DFEC methods of the present disclosure allow thecorrection of one or multiple initialization errors in a memory array.

As will be explained in detail below, the distributed flag errorcorrection (DFEC) methods of the present disclosure only correctinitialization errors (either SET or RESET, whichever was selected forthe initialization state). As such, another error correction method—suchas, but not limited to, a Bose-Chaudhuri-Hocquenghem (BCH) errorcorrection algorithm—is employed to correct the opposite type of error(programming errors), and the remaining initialization errors that DFECfailed to correct. That is, if all cells are initialized to a RESETcondition, the DFEC methods of the present disclosure would correctRESET errors and a second error correction algorithm (such as, but notlimited to, a BCH error correction algorithm) would be employed tocorrect SET errors, and remaining RESET errors. However, since the DFECmethods of the present disclosure require a very small parity overhead,and can be used to correct a percentage of the total expected errors,the required correctable bit error rate (BER) of this second errorcorrection algorithm is reduced and fewer total parity bits arerequired. This results in an overall reduction in parity overhead, asdetailed in FIGS. 16A-16C, particularly since resistive change elementarrays typically exhibit a higher rate of initialization errors thanprogramming errors, as discussed in more detail with respect to FIGS.7A-7C below.

Resistive Change Element Arrays

Resistive change cells store information through the use of a resistivechange element within the cell. Responsive to electrical stimuli, aresistive change element can be adjusted between at least twonon-volatile resistive states. Typically, two resistive states are used:a low resistive state (corresponding, typically, to a logic ‘1,’ a SETstate) and a high resistive state (corresponding, typically, to a logic‘0,’ a RESET state). In this way, the resistance value of the resistivechange element within the resistive change element cell can be used to astore a bit of information (functioning, for example, as a 1-bit memoryelement). According to other aspects of the present disclosure, morethan two resistive states may be used, allowing a single cell to storemore than one bit of information. For example, a resistive change memorycell might adjust its resistive change element between four non-volatileresistive states, allowing for the storage of two bits of information ina single cell.

Within the present disclosure the term “programming” is used to describean operation wherein a resistive change element is adjusted from aninitial resistive state to a new desired resistive state. Suchprogramming operations can include a SET operation, wherein a resistivechange element is adjusted from a relatively high resistive RESET state(e.g., on the order of 1 Me) to a relatively low resistive SET state(e.g., on the order of 100 kΩ). Such programming operations (as definedby the present disclosure) can also include a RESET operation, wherein aresistive change element is adjusted from a relatively low resistive SETstate (e.g., on the order of 100 kΩ) to a relatively high resistiveRESET state (e.g., on the order of 1 Me). Additionally, a “READ”operation, as defined by the present disclosure, is used to describe anoperation wherein the resistive state of a resistive change element isdetermined without significantly altering the stored resistive state.Within certain embodiments of the present disclosure these resistivestates (that is, both the initial resistive states and the new desiredresistive states) are non-volatile. It should also be noted that theterms program and write are used interchangeably throughout the presentdisclosure.

Though the present disclosure uses resistive change memory cells as anexample, the present disclosure is not limited to memory. Indeed, themethods of the present disclosure could be used to adjust the resistanceof resistive change elements within logic devices, analog circuitry,sensors, and the like.

Resistive change elements include, but are not limited to, two-terminalnanotube switching elements, phase change memory cells, and metal oxidememory cells. For example, U.S. Pat. No. 7,781,862 and U.S. Pat. No.8,013,363 teach non-volatile two-terminal nanotube switches comprisingnanotube fabric layers. As described in those patents, responsive toelectrical stimuli a nanotube fabric layer can be adjusted or switchedamong a plurality of non-volatile resistive states, and thesenon-volatile resistive states can be used to reference informational(logic) states. In this way, resistive change elements (and arraysthereof) are well suited for use as non-volatile memory devices forstoring digital data (storing logic values as resistive states) withinelectronic devices (such as, but not limited to, cell phones, digitalcameras, solid state hard drives, and computers). However, the use ofresistive change elements is not limited to memory applications. Indeed,arrays of resistive change elements as well as the advancedarchitectures taught by the present disclosure could also be used withinlogic devices or within analog circuitry.

FIG. 1A illustrates the layout of an exemplary resistive change cellthat includes a vertically oriented resistive change element (such astructure is sometimes termed a 3D cell by those skilled in the art). Atypical field effect transistor (FET) device 130 a is formed within afirst device layer, including a drain D, a source S, and a gatestructure 136 a. The structure and fabrication of such an FET device 130a will be well known to those skilled in the art.

A resistive change element 110 a is formed in a second device layer.Conductive structure 132 a electrically couples a first end of resistivechange element 110 a with the source terminal of FET device 130 a.Conductive structure 120 a electrically couples a second end ofresistive change element 110 with an array source line SL outside theresistive change cell. Conductive structures 134 a and 140 aelectrically couple the drain terminal of FET device 130 a with an arraybit line BL outside the resistive change cell. An array word line WL iselectrically coupled to gate structure 136 a.

FIG. 1B illustrates the layout of an exemplary resistive change cellthat includes a horizontally oriented resistive change element (such astructure is sometimes termed a 2D memory cell by those skilled in theart). A typical FET device 130 b is formed within a first device layer,including a drain D, a source S, and a gate structure 136 b. As with theFET device (130 a) depicted in FIG. 1A, the structure and fabrication ofsuch an FET device 130 b will be well known to those skilled in the art.

A resistive change element 110 b is formed in a second device layer.Conductive structure 132 b electrically couples a first end of resistivechange element 110 b with the source terminal of FET device 130 b.Conductive structure 120 b electrically couples a second end ofresistive change element 110 b with an array source line SL outside thememory cell. Conductive structures 134 b and 140 b electrically couplethe drain terminal of FET device 130 b with an array bit line BL outsidethe memory cell. An array word line WL is electrically coupled to gatestructure 136 b.

Within both of the resistive change cells depicted in FIGS. 1A and 1B,the resistive change element is adjusted between different resistivestates by applying electrical stimulus, typically one or moreprogramming pulses of specific voltages and pulse widths, between thebit line (BL) and the source line (SL). A voltage is applied to the gatestructure (136 a in FIG. 1A and 136 b in FIG. 1B) through the word line(WL), which enables electrical current to flow through the seriescombination of the FET device (130 a in FIG. 1A and 130 b in FIG. 1B)and the resistive change element (110 a in FIG. 1A and 110 b in FIG.1B). Depending on the gate voltage applied by the word line (WL),current to the resistive change element (110 a in FIG. 1A and 110 b inFIG. 1B) may be limited by design, thereby enabling the FET device tobehave as a current limiting device. By controlling the magnitude andthe duration of this electrical current, the resistive change element(110 a in FIG. 1A and 110 b in FIG. 1B) can be adjusted between aplurality of resistive states.

The state of the resistive change element cells depicted in FIGS. 1A and1B can be determined, for example, by applying a DC test voltage, forexample, but not limited to, 0.5V, between the source line (SL) and thebit line (BL) while applying a voltage to gate structure (136 a in FIG.1A and 136 b in FIG. 1B) sufficient to turn on the FET device (130 a inFIG. 1A and 130 b in FIG. 1B) and measuring the current through theresistive change element (110 a in FIG. 1A and 110 b in FIG. 1B). Insome applications this current can be measured using a power supply witha current feedback output, for example, a programmable power supply or asense amplifier. In other applications this current can be measured byinserting a current measuring device in series with the resistive changeelement (110 a in FIG. 1A and 110 b in FIG. 1B).

Alternatively, the state of the resistive change element cells depictedin FIGS. 1A and 1B can also be determined, for example, by driving afixed DC current, for example, but not limited to, 1 μA, through theseries combination of the FET device (130 a in FIG. 1A and 130 b in FIG.1B) and the resistive change element (110 a in FIG. 1A and 110 b in FIG.1B) while applying a voltage to the gate (136 a in FIG. 1A and 136 b inFIG. 1B) sufficient to turn on the FET device (130 a in FIG. 1A and 130b in FIG. 1B) and measuring the voltage across the resistive changeelement (110 a in FIG. 1A and 110 b in FIG. 1B).

The resistive change element (such as, but not limited to, thosedepicted in FIGS. 1A and 1B) can be formed from a plurality ofmaterials, such as, but not limited to, metal oxide, solid electrolyte,phase change material such as a chalcogenide glass, graphene fabrics,and carbon nanotube fabrics.

For example, U.S. Pat. No. 7,781,862 to Bertin et al., incorporatedherein by reference in its entirety, discloses a two-terminal nanotubeswitching device comprising a first and second conductive terminals anda nanotube fabric article. Bertin teaches methods for adjusting theresistivity of the nanotube fabric article between a plurality ofnonvolatile resistive states. In at least one embodiment, electricalstimulus is applied to at least one of the first and second conductiveelements such as to pass an electric current through said nanotubefabric layer. By carefully controlling this electrical stimulus within acertain set of predetermined parameters (as described by Bertin in U.S.patent application Ser. No. 11/280,786) the resistivity of the nanotubearticle can be repeatedly switched between a relatively high resistivestate and relatively low resistive state. In certain embodiments, thesehigh and low resistive states can be used to store a bit of information.

As described by the incorporated references, a nanotube fabric asreferred to herein for the present disclosure comprises a layer ofmultiple, interconnected carbon nanotubes. A fabric of nanotubes (ornanofabric), in the present disclosure, e.g., a non-woven carbonnanotube (CNT) fabric, may, for example, have a structure of multipleentangled nanotubes that are irregularly arranged relative to oneanother. Alternatively, or in addition, for example, the fabric ofnanotubes for the present disclosure may possess some degree ofpositional regularity of the nanotubes, e.g., some degree of parallelismalong their long axes. Such positional regularity may be found, forexample, on a relatively small scale wherein flat arrays of nanotubesare arranged together along their long axes in rafts on the order of onenanotube long and ten to twenty nanotubes wide. In other examples, suchpositional regularity maybe found on a larger scale, with regions ofordered nanotubes, in some cases, extended over substantially the entirefabric layer. Such larger scale positional regularity is of particularinterest to the present disclosure. Nanotube fabrics are described inmore detail in U.S. Pat. No. 6,706,402, which is included by referencein its entirety.

While some examples of resistive change cells and elements within thepresent disclosure specifically reference carbon nanotube basedresistive change cells and elements, the methods of the presentdisclosure are not limited in this regard. Indeed, it will be clear tothose skilled in the art that the methods of the present disclosure areapplicable to any type of resistive change cell or element (such as, butnot limited to, phase change and metal oxide).

Referring now to FIG. 2, an exemplary architecture for a resistivechange element memory array 200 is illustrated in a schematic diagram.The array 200 comprises a plurality of cells (CELL00-CELLxy), each cellincluding a resistive change element (SW00-SWxy) and a selection device(Q00-Qxy). The individual array cells (CELL00-CELLxy) within resistivechange array 200 are selected for reading and programming operationsusing arrays of source lines (SL[0]-SL[x]), word lines (WL[0]-WL[y]),and bit lines (BL[0]-BL[x]) as will be described below.

Within the exemplary architecture of FIG. 2, the selection devices(Q00-Qxy) used with the individual array cells (CELL00-CELLxy) areconventional silicon based FETs. However, such arrays are not limited inthis regard. Indeed, other circuit elements (such as, but not limitedto, diodes or relays) could be used within similar architecturestructures to provide cell selection functionality within an array (forexample, selection devices such as bipolar devices, and FET devices suchas SiGe FETs, FinFETs, and FD-SOI).

FIG. 3 is a table 300 describing exemplary programming and READoperations for the resistive change element array 200 shown in FIG. 2.The table lists the word line, bit line, and source line conditionsrequired to perform a RESET operation, a SET operation, and a READoperation on CELL00 of resistive change element array 200. Theseoperations as well as the function of the resistive change element array200 depicted in FIG. 2 within these operations will be described indetail below.

The first column of the table within FIG. 3 describes a RESET operationof CELL00—that is, a programming operation which adjusts the resistivestate of resistive change element SW00 from a relatively low resistivestate to a relatively high resistive state. WL[0] is driven to V_(PP),the logic level voltage required to enable select device Q00, while theremaining word lines WL[1:y] are driven to 0V (essentially grounded). Inthis way, only the select devices in the first row of the array,Q00-Qx0, are enabled (or, “turned on”). BL[0] is driven to V_(RST) (theprogramming voltage level required to RESET element SW00), and SL[0] isdriven to 0V. The remaining bit lines, BL[1:x], and the remaining sourcelines, SL[1:x], are held in high impedance states. In this way V_(RST)is driven across only the cells in the first column of the array,CELL00-CELL0 y, while V_(PP) is applied across only WL[0], enabling thefirst row of the array, CELL00-CELLx0. As a result of these conditions,the programming voltage V_(RST) is driven only across SW00 to adjust itsresistive state. The other select devices within the array remainisolated from the programming voltage and thus retain their originallyprogrammed resistive state.

The second column of the table within FIG. 3 describes a SET operationof CELL00—that is, a programming operation which adjusts the resistivestate of resistive change element SW00 from a relatively high resistivestate to a relatively low resistive state. As with the RESET operation,WL[0] is driven to V_(PP), while the remaining word lines, WL[1:y], aredriven to 0V (essentially grounded). In this way, only the selectdevices in the first row of the array, Q00-Qx0 are enabled (or “turnedon”). SL[0] is driven to V_(SET), the programming voltage level requiredto drive SW00 into a relatively low resistive state, and BL[0] is drivento 0V. The remaining source lines, SL[1:x], and the remaining bit lines,BL[1:x], are held in high impedance states. In this way, V_(SET) isdriven across only the cells in the first column of the array,CELL00-CELL0 y, and V_(PP) is driven across only WL[0], enabling thefirst row of the array, CELL00-CELLx0. As a result of these conditions,the programming voltage V_(SET) is driven only across SW00 throughenabled selection device Q00, changing its resistive state to one of lowimpedance. The other select devices within the array remain isolatedfrom the programming voltage and thus retain their originally programmedresistive state.

Finally, the third column of the table within FIG. 3 describes a READoperation of CELL00, that is, an operation which determines theresistive state of resistive change element SW00. As with the SET andRESET operations, WL[0] is driven to V_(PP), the programming voltagerequired to enable select device Q00, while the remaining word linesWL[1:y] are held at low voltage (approximately 0V in this example) sothat only the select devices in the first row of the array Q00-Qx0 areenabled (or “turned on”). SL[0] is driven to V_(RD), the voltage levelrequired to READ the resistive state of SW00, and BL[0] is driven to 0V(essentially grounded). The remaining source lines SL[1:x] and theremaining bit lines BL[1:x] are held in high impedance states. In thisway, V_(RD) is driven across only the cells in the first column of thearray, CELL00-CELL0 y, and V_(PP) is driven across only WL[0], enablingthe first row of the array, CELL00-CELLx0. As a result of theseconditions, the READ voltage V_(RD) is driven only across SW00, whilethe other select devices within the array remain isolated from the READvoltage. In this way, current will flow only through resistive changeelement SW00, and by measuring that current, the resistive state of SW00can be determined.

It should be noted that the programming voltages for the RESET and SEToperations (V_(RST) and V_(SET), respectively) as described in thepreceding paragraphs were applied in opposite polarities. However, themethods of the present disclosure are not limited in this regard.Indeed, the differing polarity of the RESET and SET operations were usedin order to better illustrate the functionality of the array depicted inFIG. 2. That is to say, programming (SET and RESET) voltage and READvoltages can be driven in either polarity (that is, positive voltage onthe source line or positive voltage on the bit line) dependent upon theneeds of the specific type of resistive change element used or thespecific programming operation in question. Also, within FIG. 3 thevoltage level required to enable the select devices is listed as systemvoltage V_(PP) for ease of explanation. However, it should be noted thatsuch a voltage can be, in certain applications, less than a supply railor other system level voltage depending on the type and requirements ofa specific select device and the needs of a given application. As such,the use of V_(PP) within table 300 is intended as a non-limiting exampleonly.

Referring now to FIG. 4, a second exemplary architecture for a resistivechange element array 400 is illustrated in a simplified schematicdiagram. Within the exemplary architecture 400, no selection devices orother current limiting elements are used within the resistive changeelement cells. That is, each cell is comprised only of a resistivechange element that is accessed via two control lines (a word line and abit line). Such an architecture is sometimes referred to as a 1-R arrayby those skilled in the art.

As with the array architecture 200 detailed in FIG. 2, the arrayarchitecture 400 of FIG. 4 can address individual resistive change cellswithin the array by driving the word lines and bit lines with a specificbias. In the absence of any selection devices within the individualarray cells (CELL00-CELLxy), an access operation to array architecture400 must provide a sufficient electrical stimulus—as required for aprogramming (SET or RESET) or READ operation—to a selected array celland, at the same time, prevent the other cells in the array fromexperiencing any electrical stimuli that would alter their storedresistive state. Methods and circuits for programming and accessingcells within 1-R array architectures (as depicted in FIG. 4) aredisclosed in U.S. Pat. Nos. 9,263,126 and 9,299,430, both of which areincorporated herein by reference in their entireties.

For example, to access CELL00 within array architecture 400 of FIG. 4, asufficient READ, SET, or RESET voltage (or current) is applied to WL[0]while BL[0] is driven to ground (0V). The remaining word lines(WL[1]-WL[y]) and the remaining bit lines (BL[1]-BL[x]) are driven athalf the voltage (or current) supplied to WL[0]. In this way, only halfof an applied programming or READ voltage (or current) is applied to theresistive change elements within the remaining cells on the selected bitline (BL[0])—that is, CELL01-CELL0 y—and within the remaining cells onthe selected word line (WL[0])—that is, CELL10-CELLx0. That is,CELL01-CELL0 y each see half of the applied programming or READ voltageon their associated word line and 0V on their associated bit lines, andCELL10-CELLx0 see the full programming or READ voltage on theirassociated word lines but only half the programming or READ voltage ontheir associated bit line. The remaining cells in the array—that is,CELL11-CELLxy—are unbiased, each of those cells seeing half of theapplied programming or READ voltage (or current) on both its associatedword line and on its associated bit line, resulting in no voltage dropor current flow across/through the resistive change elements in thosecells. In this way, the applied programming or READ voltage is appliedonly over the selected resistive change element SW00, and while some ofthe unselected cells within the array are partially biased during theaccess and addressing operation, the electrical stimuli applied to thosecells is not sufficient to alter the resistive state of those cells ordisturb the programming or READ operation being performed on theselected cell.

FIG. 5 is a perspective drawing of a 3D 1-R resistive change elementarray 500. Resistive change element array 500 is comprised of 1-Rresistive change cells arranged in three dimensions (along the x-, y-,and z-axes). A first layer of bit lines (542 a, 544 a, 546 a, and 548 a)are disposed along the y-axis, and a first layer of word lines (532 a,534 a, 536 a, and 538 a) are disposed along the x-axis and above thisfirst layer of bit lines. Between these first two layers of bit lines(542 a, 544 a, 546 a, and 548 a) and words lines (532 a, 534 a, 536 a,and 538 a), a first layer of resistive change elements 510 is disposed,one resistive change element at each word line and bit line crossing.The resistive change elements are each comprised of a resistive changematerial 516 (such as, but not limited to, a nanotube fabric layer or ablock of phase change material) disposed between a first conductiveelement 512 and a second conductive element 514. It is desirable, incertain applications, to use these first and second conductive elements(512 and 514, respectively) to provide a conductive path between anarray line (a word or bit line) and the actual resistive change material516. However, these conductive elements (512 and 514) are not requiredin every application. For example, depending on the material used forthe array lines, the particular material selected for the resistivechange element 516, and the layout and fabrication methods being used,in certain applications it could be more favorable for the resistivechange material block to connect directly to the array lines themselves.As such, the inclusion of first and second conductive elements (512 and514, respectively) should not be seen as limiting with respect to thearchitecture of 1-R resistive change element arrays.

A second layer of bit lines (542 b, 544 b, 546 b, and 548 b) is disposedalong the y-axis above the first layer of word lines. Between thissecond layer of bit lines (542 b, 544 b, 546 b, and 548 b) and the firstlayer of words lines (532 a, 534 a, 536 a, and 538 a), a second layer ofresistive change elements 510 is disposed, one resistive change elementat each word line and bit line crossing. A second layer of word lines(532 b, 534 b, 536 b, and 538 b) is disposed along the x-axis above thesecond layer of bits lines (542 b, 544 b, 546 b, and 548 b), and a thirdlayer of resistive change elements 510 is disposed, one resistive changeelement at each word line and bit line crossing. In this way, an arrayof forty-eight 1-R resistive change element cells is arranged withinessentially the same cross sectional area that would be used for anarray of only sixteen array cells within a traditional 2D arraystructure.

Arrays of non-volatile, reprogrammable resistive change element arrays(such as those detailed in FIGS. 2, 4, and 5) are highly desirablewithin certain applications, for example in solid state drives (SSDs) oras replacements for silicon based flash memory. As technology increases,resistive change elements are scaling smaller, and more compact, denserarrays of such elements are being developed. As such, an improved errorcorrection method that takes advantage of the programmingcharacteristics of resistive change elements (as described in detailabove) to improve error correction latency and reduce parity celloverhead is highly desirable. To this end, the distributed flag errorcorrection (DFEC) methods of the present disclosure is well suited forsuch applications.

FIG. 6 is a graph 600 plotting parity overhead against correctable biterror rate (BER) for three difference sized memory arrays (512 byte, 1kilobyte, and 2 kilobyte) using a conventionalBose-Chaudhuri-Hocquenghem (BCH) error correction algorithm. BCH codesare well known by those skilled in the art and form a class of cyclicerror-correcting codes that are constructed using finite fields. BCHerror correction is commonly used in electronics applications (such as,but not limited to, flash memory, solid state drives, and digital media)and is particularly useful as there is a precise control over the numberof errors correctable by the code. In particular, it is possible todesign binary BCH codes that can correct multiple bit errors. Using aBCH error correction algorithm, the number of parity bits “m”theoretically required to correct one bit error within “n” bits (userdata bits plus overhead bits) is given by:

n≦2^(m)−1  (EQ1)

For example, 13 BCH parity bits are required to find a single error in ablock with 512 bytes (4096 bits) of user data bits plus overhead bits,since 2¹²−1=4095<4096 and 2¹³−1=8191≧4096. The overhead bits include BCHparity bits for error correction. The number of overhead bits is muchless than the number of data bits so that the total number of bits,including 512-bytes of data bits plus overhead bits, is less than 8191.Similarly, it would require 14 BCH parity bits to find a single error ina block containing 1 kilobyte (8192 bits) of data bits plus overheadbits, and 15 BCH parity bits to find a single error in a blockcontaining 2 kilobytes (16,384 bits) of data bits plus overhead bits. Inorder to realize a 0.1% correctable bit error rate (BER) in a memoryblock with 512-bytes of data bits (4096 bits), an error correctionalgorithm would need to be able to correct four bit errors(0.1%*4096≈4), which would require 52 BCH parity bits (4*13=52). Usingsimilar calculations, 112 BCH parity bits are required to realize a 0.1%correctable BER in a block containing 1 kilobyte of data bits plusoverhead bits, and 240 BCH parity bits are required to realize a 0.1%correctable BER in a block containing 2 kilobytes of data bits plusoverhead bits. The number of BCH parity bits increases for highercorrectable BERs. For example, a correctable BER of 0.2% on a block with512 bytes of user data would require 104 BCH parity bits. According tothe present disclosure, the number of parity bits within a block ofmemory can be used to define the parity overhead, given by thefollowing:

Parity Overhead=Parity Bits/Data Bits  (EQ2)

Using EQ2, it can be seen that using a BCH algorithm to realize a 0.1%correctable BER on a memory block with 512 bytes of data bits and paritybits results in a parity overhead of approximately 1.3% (52 paritybits/(4096 data bits+52 parity overhead bits+other overhead bits)). Torealize a 0.2% correctable BER on the same 512 byte memory block resultsin a parity overhead of 2.5% (104 parity bits/4096 data bits). As shownin FIG. 6, when using a BCH error correction method alone parityoverhead increases linearly with increasing correctable BER, approaching4% parity overhead as correctable BER moves past 0.3% for 512 bytes, 1kilobyte, and 2 kilobytes of data bits. In certain applications, thisincreased parity overhead results in increased latency within thedecoding process for the error correction algorithm as well as usingincreasingly more surface area on a chip. To this end, it would behighly desirable to achieve a required correctable BER with a lowerparity overhead, providing a resistive change element array withincreased storage capacity, faster operating speed, and a more compactsurface area.

The above calculation of correctable BER is valid only for one memoryblock in which a BCH ECC operates. In a memory array with multiplememory blocks, the correctable BER is much less than this calculatednumber due to variation of error occurrence among different ECC blocks.For example, if a three-block memory array is manufactured such that onememory block has a BER of 0.3% while the other two experience zeroerror, the averaged correctable BER would be 0.1%. However, since thelocation of a manufacturing defect cannot be predicted, a parityoverhead of 4% must be used on all three memory blocks.

FIG. 7A is a plot 701 characterizing the error rates of an exemplaryresistive change element array. The exemplary array used to generate thecharacterization data in FIG. 7A (as well as FIGS. 7B and 7C, discussedbelow) was a 256-byte array of non-volatile two-terminal nanotubeswitches comprising nanotube fabric layers, as are taught in U.S. Pat.No. 7,781,862 and U.S. Pat. No. 8,013,363 and described in detail withrespect to FIGS. 1A and 1B above. The exemplary array was writtenthrough 10⁸ write cycles and the total number of errors observed wasplotted in FIG. 7A. It should be noted that FIGS. 7A, 7B, and 7C plotthe number of errors observed with normalized arbitrary units (a. u.)for ease of explanation. Characterizing a resistive change element arrayin this way provides a statistical model of the number of errors thatcan be expected during write operations to the array and the correctableBER required within an error correction method designed for the array.All three plots (701, 702, and 703) use the same a.u. scale for they-axis so that the relative error rates between plots can be compared.

Looking now to FIGS. 7B and 7C, the errors plotted in FIG. 7A are brokenout into plots showing the RESET errors (plot 702 in FIG. 7B) and SETerrors (plot 703 in FIG. 7C). As described above, a RESET error,according to the methods of the present disclosure, occurs when aprogramming operation attempts to adjust a resistive change element intoa relatively high resistance RESET state (a logic “0”) but the cellremains in a relatively low resistance SET state (a logic “1”).Conversely, a SET error, according to the methods of the presentdisclosure, occurs when programming operation attempts to adjust aresistive change element into a relatively low resistance SET state (alogic “1”) but the cell remains in a relatively high resistance RESETstate (a logic “0”). As can be seen comparing FIGS. 7B and 7C, theexemplary resistive change element array characterized within plots 701,702, and 703 possesses a significant bias toward RESET errors. That is,when programming this exemplary array, significantly more RESET errorscan be expected to occur as compared to SET errors. In this particularexample, at 10⁸ write cycles, it was found that 80% of the total errors(as plotted in FIG. 7A) were RESET errors and 20% were SET errors. Thischaracteristic bias toward RESET errors makes this exemplary array wellsuited for the distributed flag error correction (DFEC) methods of thepresent disclosure, as will be explained in more detail below.

In addition to the bias toward one type of error, resistive changeelement arrays also exhibit other characteristics that make them wellsuited for the DFEC methods of the present disclosure. For example,programming errors within resistive change elements tend to be randomlydistributed across the array. That is, unlike, for example, siliconbased memory technologies, programming errors within resistive changeelements typically do not cluster around a certain region or area of thearray and are generally spread uniformly across the array. Additionally,programming errors within resistive change element arrays tend to betemporary and repairable, or, “soft” errors, whereas programming errorsin silicon based memories, for example, are typically caused by damagedor otherwise broken cells, referred to as “hard” errors. That is, sometypes of resistive change elements, such as NRAM, that exhibit a RESETerror can be corrected by simply repeating a programming operation, or,more commonly, by taking the cell through a full SET/RESET cycle. Aswill be described in detail below, the DFEC methods of the presentdisclosure take advantage of these characteristics of resistive changeelement arrays to provide error correction methods with reduced parityoverhead—and, by extension, improved latency and smaller chip surfacearea—as compared with conventional error correction techniques.

Further, as will be described in detail within the discussion of FIGS.10 and 11A-11E below, the DFEC methods of the present disclosure canprogram—that is, write data to—a resistive change element array using atwo step process. In a first process step, all of the cells within ablock or subsection of a resistive change element array are initializedto either a RESET or a SET state, as befits the needs of a particularapplication. As described above, the selection of the initializationstate for an array is typically dependent on which state can be expectedto exhibit a greater percentage of errors. After this initializationprocess adjusts all cells within the block or subsection into a singlestate, only those cells that are required to store a data value oppositeof the initialization state are programmed in a second operation. Forexample, for an array wherein the initialization condition was RESET, afirst programming operation would RESET all of the cells within a block.Then a second programming operation would then SET only those cells thatrequire a logic “1” according to the data being written, as those cellsthat require a logic “0” are already in the desired state.

FIG. 8A is a diagram illustrating an exemplary resistive change elementarray 801 configured to use a conventional BCH error correction methodalone. The array 801 is divided into eight blocks 820. FIG. 8Billustrates these blocks 820 in more detail in diagram 802. Errors 850within the array 801 are indicated with an X. As can be seen in bothFIGS. 8A and 8B, each of the blocks 820 include a region of data cells830 and a region of BCH parity cells 840. The number of parity cells 840is determined by the correctable BER required for the array and EQ1 (asdiscussed in detail above). For example, if FIG. 8A represented an arrayincluding 8-kilobyte (65,536 bit) data bits and overhead bits, and eachblock has a maximum correctable BER of 0.1%, each block would include adata region 830 with 8,192 data cells and a parity region 840 with8×14=112 BCH parity cells. In this example, the ratio of parity cells todata cells within each block 820 would be 1.4%, as given by EQ2 anddiscussed in detail above, representing the parity overhead of theblock.

FIG. 8C is a table 803 detailing the number of BCH parity bits (840 inFIGS. 8A and 8B) and the parity overhead required to achieve a specifiedcorrectable BER using only BCH error correction for a resistive changeelement array with 8 kilobytes of data bits configured as shown in FIG.8A. For instance, looking to the first row of table 803, a correctableBER of 0.1% would require the BCH error correction algorithm to find 8errors in each block containing 1 kilobyte of data bits (8192 bits), asshown in the second column of table 803. According to EQ1, 14 BCH paritybits are required to find a single error in a 1 kilobyte block, so 112BCH parity bits are required to find 8 errors (14 parity bits×8 errors),as shown in the third column of table 803. A block with 112 BCH paritybits and 8192 data bits, as described above, results in a parityoverhead (given by EQ2) of 1.4%. As detailed in table 803, similarcalculations show that for 0.2%, 0.3%, 0.4%, and 0.5% correctable BER onthe resistive change element array configured as detailed in FIG. 8A,parity overheads of 2.9%, 4.4%, 6.0%, and 7.5% are required,respectively. As will be shown in at least FIGS. FIGS. 16A-16C, the DFECmethods of the present disclosure may significantly reduce these parityoverheads while maintaining the required correctable BER.

Reverse Flag Error Correction

As discussed previously, the simplest DFEC method of the presentdisclosure, the reverse-flag error correction (RFEC) method, distributesflag bits across a resistive change element array to delimitate eachDFEC subsection with a single flag cell. This distributed flag can beused to indicate the presence of at least one initialization error inits subsection and thereby match the input data to the error pattern.The use of a single flag bit allows the input data to be matched to theerrors according to two patterns—no effective errors (flag bit is notactivated), and at least one effective error (flag bit is activated), aswill be described in detail with respect to FIGS. 9A-9C, 10, and11A-11E. In this way, the single-flag DFEC method of the presentdisclosure may be guaranteed to correct at least one initializationerror per subsection.

Looking now to FIG. 9A, this diagram illustrates a resistive changeelement array 901 configured to use the RFEC method of the presentdisclosure in concert with a BCH error correction method. As with array801 shown in FIG. 8A, array 901 is divided into eight blocks 910, whichare illustrated in more detail in FIG. 9B. As can be seen in both FIGS.9A and 9B, each block 910 is further divided into four subsections 920.Each of these four subsections 920 includes a region of data cells 920 aand a single flag cell 920 b for storing a single DFEC flag bit. Eachblock also includes a region of BCH parity cells 930 configured tocorrect errors across the entire block 910. Errors may occur across thearray, including data bit errors 940, flag bit errors 950, and BCHparity bit errors 960. As will be discussed in detail below with respectto FIG. 10, flag cells 920 b provide the ability to correct at least oneinitialization error within the data bits 920 a of each subsection 920,reducing the total number of errors required to be corrected by the BCHerror correction algorithm and, in turn, the number of BCH parity cells930 required for each block 910. As will be shown in FIG. 9C, thisconfiguration (one flag cell per subsection combined with a reducednumber of BCH parity cells) provides a lower overall parity overhead forthe array 901 as compared with using BCH error correction alone (as isshown in FIGS. 8A and 8B).

As described above, and in more detail with respect to FIG. 10 below,the flag bits used in the DFEC methods of the present disclosure areonly capable of correcting initialization errors within an array. Thatis, for a resistive change element array that initializes cells into aRESET state during a programming operation, the flag bits will correctonly RESET errors within each subsection. And, similarly, for aresistive change element array that initializes cells into a SET stateduring a programming operation, the flag bits will only correct SETerrors within each subsection. Further, the DFEC methods of the presentdisclosure may only correct initialization errors 940 in data bits 920a. BCH parity bit errors 960 and flag bit errors 950, as well asprogramming errors, must be corrected by a secondary error correctionmethod. As discussed previously, in certain applications wherein aresistive change element array tends to bias toward one type of error(SET or RESET), the type of error with greater occurrence should beselected as the initialization condition, as a higher number of errorscan be expected to be corrected using the flag cell within eachsubsection (as will be shown in detail within the discussion of FIG. 10below).

To this end, for an exemplary resistive change element array configuredas shown in FIG. 9A, at the 10⁸th write cycle, 75% of the errors thatoccur may be expected to be RESET errors and 25% may be expected to beSET errors (such an error distribution is in line with the exemplaryerror rate characterization shown in FIGS. 7A-7C and discussed in detailabove). Such an error distribution would mean that the majority oferrors which occur are RESET errors, and thus the flag cells 920 b couldbe expected to correct a significant portion of the total errors withintheir respective subsection. Specifically, each flag cell can beguaranteed to correct at least one RESET error per subsection 910,should a RESET occur. Effectively, this means that in an ideal case oneerror bit can be corrected by one flag cell 920 b. This is much moreefficient than using BCH alone, which needs 14 parity bits to correctone error. Flag bit cells 920 b can therefore be used to reduce thenumber of bits 930 that a BCH error correction routine will require toachieve a given correctable BER and, in turn, reduce the parity overheadwithin array 901.

In real applications, however, RESET errors are not evenly distributedbetween subsections. Some subsections, such as 970, may not have a RESETerror at all, while others, such as 980, may contain multiple errors. Asa result, the flag bits in subsections with no errors may beunnecessary, while in subsections with multiple errors the flag bits maybe unable to correct all of the errors. The number of flag bits 920 b,and by extension the number and size of subsections 920, shouldtherefore be carefully determined based on the RESET error rate, as willbe described in more detail in FIGS. 13A-13D and 16A-16C. If too manyflag bits are used, many subsections may not have a RESET error and theparity overhead may be unnecessarily large. On the other hand, somesubsections may have multiple reset errors if too few flag bits are usedor the RESET error rate is high. In this case, additional flag bits 920b may be used to reduce the subsection size and separate the RESET errorbits into different subsections, thereby reducing the reliance on BCHcorrection.

FIG. 9C details this reduction in parity overhead for the resistivechange element array configured as detailed in FIG. 9A for a correctableBER of 0.1%. Looking to the first row of table 903, a correctable BER of0.1% would require an error correction algorithm to find 8 errors ineach block with 1 kilobyte (8192 bit) of data bits plus overhead bits,as shown in the second column of table 903. In this analysis, assumingfour flag bits 920 b in each block 910 (one per subsection 920) can bestatistically expected to correct 3 errors due to the errordistribution. This means that a BCH error correction routine will onlyneed to correct 5 additional errors, which due to set errors andmultiple RESET errors in one subsection, to achieve the required 0.1%correctable BER (8 total errors over the 1 kilobyte block). Aspreviously discussed with respect to FIG. 8C above, 14 BCH parity bitsare required to correct a single error in the same 1-kilobyte block (asindicated by EQ1). As such, only 70 BCH parity cells are required tofind the remaining 5 errors compared to the 112 BCH parity cellsrequired for the BCH only configuration shown in FIG. 8C. This giveseach block 910 a total of 74 parity cells (70 BCH parity cells 930 and 4flag cells 920 b) and a total parity overhead of 0.9% for a 0.1%correctable BER compared to 112 parity cells and 1.4% parity overhead inthe BCH only configuration detailed in FIG. 8C.

FIG. 10 is a flow chart illustrating the single-flag DFEC method of thepresent disclosure used during a programming/read back operation 1000performed on a single subsection of a resistive change element array(analogous to subsection 920 in array 901 within FIG. 9A). It should benoted that within FIG. 10 (as well as within the examples presentedwithin FIGS. 11A-11E, 16, and 17A-17F further below) a RESET state isselected as the initialization state for ease of explanation. However,the methods of the present disclosure are not limited in this regard. Asdescribed in detail above, the DFEC methods of the present disclosurecan be used to correct either SET or RESET errors, as befits the needsof a particular application. As such, the examples of FIGS. 10 and11A-11E (as well as those throughout the present disclosure), whereinthe DFEC methods of the present disclosure are used to correct RESETerrors within a resistive change element array that initializes cells toa RESET state during a programming/read back operation, should beconsidered non-limiting examples presented for illustrative purposesonly.

Looking again to FIG. 10, within a first process step 1012 all of thecells within the subsection are initialized into a RESET condition. In asecond operation 1014, the subsection is checked for any RESET errors(that is, the subsection is checked to ensure that no cells failed to beinitialized into a relatively high resistive RESET state, correspondingto a logic “0”). Looking now to process step 1016, if there are no cellsin the subsection in a SET state (that is, if there are no RESET errorsafter initialization), the programming/read back operation 1000 advancesto process step 1050 wherein the input data to be programmed to thesubsection is encoded with a BCH ECC algorithm. However, if at least onecell within the subsection remains uninitialized, programming/read backoperation 1000 advances to data pattern matching (DPM) step 1022.

Within DPM step 1022, the data values required to be written to thesubsection during programming/read back operation 1000 are checked tosee if the uninitialized cell (or cells) is due to be programmed intoeither a SET or a RESET state. Within process step 1024, if the errorcell (or all of the error cells, in the case of multiple initializationerrors within the subsection) is due to be programmed into a SETcondition there is effectively no error, and the programming/read backoperation 1000 advances to process step 1050 wherein the input data tobe programmed to the subsection is encoded with a BCH ECC algorithm. Inthis case, wherein the erroneously uninitialized cell—or cells—is due tobe programmed into the opposite of the initialization state (a SET statein this example), there is effectively no error, and the uninitializedcell can be left alone or programmed more completely into a SET statewithin process state 1060.

However, if at least one effective error has occurred, wherein at leastone cell is due to be programmed into a RESET state (as per the inputdata supplied to the programming/read back operation 1000), theprogramming/read operation 1000 advances to process step 1030 whereinthe subsection's flag bit (960 b in FIGS. 9A and 9B) is activated (setto a logic “1” in this example). As will be shown further below, theflag bit will be used in reverse data pattern matching (RDPM) step 1082to decode the DFEC programmed data and recover at least one RESET errorwithin the subsection. In some cases, however, wherein the number ofeffective initialization errors is less than half of the number of totalinitialization errors, the flag cell may remain un-activated. In suchcases, inverting the input data will result in a larger number of errorsthan if it remains un-inverted. This situation may be detected duringthe DPM step to ensure the maximum number of errors are corrected. Oncethe flag bit is activated (in process step 1030), the programming/readback operation 1000 advances to DFEC encoding step 1040 wherein theinput data due to be programmed into the subsection is inverted. Thatis, the input data due to be programmed into the subsection is logicallyinversed, such that logic “1” input data values become logic “0” andlogic “0” input data values become logic “1”. The programming/read backoperation 1000 then advances to process step 1050 wherein the data to beprogrammed to the subsection, now inverted, is encoded with a BCH ECCalgorithm.

It should be noted that in the present disclosure an “effective error”or “effective initialization error” may be used interchangeably to referto a data cell which failed to initialize during an initialization stepand is due to be programmed into an initialized state according to a setof input data. That is, for example, if a RESET state is selected as theinitialization state, an effective error is a data cell which remains ina SET state following initialization of the array and is due to beprogrammed to a RESET state as per the input data. By contrast, aninitialization error may also occur wherein a data cell fails toinitialize into a selected initialization state (RESET, for instance)and remains in the opposite logical state (SET, for instance), butaccording to the input data is already in the correct state (that is,the cell was due to be programmed to a SET state, for instance). In thiscase there is an initialization error but not an effective error, as theerror may not need to be corrected.

Next, within process step 1060, all of the cells that require a logic“1”—as per either the original input data supplied to theprogramming/read back operation 1000 or the inverted version of thatdata—are programmed with a SET operation, adjusting those cells into arelatively low resistance state corresponding to a logic “1”. Withinprocess step 1060, all of the cells that require a logic “1” (that is,the data cells within the subsection, the flag cell, and the BCH paritycells—as shown in FIGS. 9A and 9B) can, in certain applications, beprogrammed (that is, adjusted into a SET state) within a singleoperation. In this way, the DFEC encoded input data is further processedusing the BCH error correcting code (BCH ECC) and written to thesubsection. At this point the subsection has been successfullyprogrammed and the programming operation ends

In a next process step 1070, the programmed data is read out from thememory array and first processed through a BCH ECC decoding algorithm.Once corrected by BCH ECC, the subsection's flag bit (920 b in FIGS. 9Aand 9B) is checked in reverse data pattern matching (RDPM) step 1082. Ifthe subsection's flag bit is not activated (that is, if the bit was notactivated in process step 1030) then the read operation ends in step1090, as no further processing needs to be done for the data bits in thesubsection. However, if the subsection's flag bit is activated (that is,if the bit was activated in process step 1030), then read operation 1000is advanced to DFEC decoding step 1084 wherein all of the bits insubsections with an activated flag bit are inverted to reflect theactual data values before the data inversion step 1040.

As previously discussed, the RFEC method of the present disclosure iscapable of correcting at least one initialization error within asubsection. However, the RFEC method may not be able to correct all ofthe initialization errors in a subsection if it contains multipleerrors, depending on the error locations and the user data pattern (aswill be discussed in detail within the discussion of FIGS. 11D and 11Ebelow). Further, as previously discussed, the DFEC methods of thepresent disclosure are only capable of correcting initialization errors(as occur in process step 1012) and cannot correct programming errorsthat occur in process step 1060. As such, an additional error correctionprocess is required when using the DFEC methods of the presentdisclosure to correct any programming errors that occurred withinprocess step 1060, as well as any initialization errors not corrected bythe DFEC methods of the present disclosure in process steps 1030 and1040. To this end, programming/read back operation 1000 performs a BCHerror correction cycle on the subsection—via process steps 1050 and1070—to correct any programming errors that occur within process step1060, as well as any initialization errors not corrected within processsteps 1030 and 1040.

It should be noted that while FIG. 10 (as well as the examples detailedin FIGS. 11A-11E) makes use of a BCH error correction method, thepresent disclosure is not limited in this regard.

FIGS. 11A-11E detail several examples of the RFEC method of the presentdisclosure used within different programming/read back operations(1101-1105) on a resistive change element array subsection (analogous to920 in FIGS. 9A and 9B) that includes four data cells (analogous to 920a in FIGS. 9A and 9B) and one flag cell (analogous to 920 b in FIGS. 9Aand 9B). It should be noted that in real applications the number of bitsin each subsection is much larger than 4. The examples within FIGS.11A-11E describe a resistive change element array configuration whereincells are initialized into a relatively high resistance RESET state(corresponding to a logic “0”), however, as previously discussed, themethods of the present disclosure are not limited in this regard. Theexemplary programming operations in FIGS. 11A-11E are intended asnon-limiting illustrative examples of the RFEC method of the presentdisclosure (as detailed in FIG. 10 and discussed above) applied todifferent types of error conditions with a resistive change elementarray. Further, the exemplary programming operation within FIGS. 11A-11Emake use of a BCH error correction cycle (within programming steps 1150a-1150 e and 1170 a-1170 e).

Looking now to FIG. 11A, the RFEC method of the present disclosure isused to correct a single initialization error (RESET error) on a datacell intended to be programmed into a RESET state. At the start ofexemplary programming operation 1101, a first programming step 1110 a(analogous to operations 1012, 1014, and 1016 in FIG. 10) attempts toinitialize all of the bits within the array subsection (all originallyin unknown logic states represented by an “X”) into a RESET state (logic“0”). However, within this example, the second cell within the arraysubsection fails to initialize, introducing RESET error 1115 a into thesubsection. In a next programming step 1120 a (analogous to operations1022 and 1024 in FIG. 10), the input data 1125 a (that is, the data dueto be written into the subsection) is checked to see if theuninitialized cell is due to be programmed into a SET or RESET state. Asthe second cell requires a logic “0” (that is, a RESET state), theuninitialized cell needs to be corrected using the DFEC method of thepresent disclosure.

To correct RESET error 1115 a, programming step 1130 a (analogous tooperation 1030 in FIG. 10) activates the subsection's flag bit, and DPMprogramming step 1140 a (analogous to operation 1040 in FIG. 10) invertsinput data 1125 a. Within this non-limiting example, the inverted dataand activated flag bit are initially stored in a chip cache tofacilitate BCH ECC processing in programming step 1150 a. Withinprogramming step 1160 a (analogous to operation 1060 in FIG. 10), theinverted input data is programmed into the array. As shown in FIG. 11A,programming step 1160 a adjusts the fourth cell of the subsection intoSET state, according to the inverted input data. As the second cellwithin the subsection is already in a SET state (due to RESET error 1115a), within certain applications it may not be necessary to perform a SEToperation on this cell within programming step 1160 a. Within suchoperations, the uninitialized cell is already within a valid and stableSET state and no further adjustment to the cell is required. In otherapplications, a SET operation may be performed on the second cell withinprogramming step 1160 a to ensure the cell is fully adjusted into a SETstate. In either case, the SET condition stored within the second cellof the subsection is now correctly programmed according to the inverteddata, correcting RESET error 1115 a.

In a next programming step 1170 a, the programmed data is read back outof the subsection and processed through a BCH ECC decoding algorithm(analogous to operation 1070 in FIG. 10). Since there are no othererrors within this example 1101, the data values within the subsectionremain unchanged. As the flag bit is activated (that is, programmed to alogic “1” within this exemplary programming/read back operation 1101),in RDPM programming step 1180 a (analogous to operations 1082 and 1084in FIG. 10) all of the bits within the subsection are inverted. In thisway, the RESET error 1115 a introduced during the subsectioninitialization in programming step 1110 a is corrected using a singleparity bit.

FIG. 11B demonstrates the single-flag DFEC method of the presentdisclosure when a single initialization error (RESET error) occurs on adata cell intended to be programmed into a SET state. At the start ofexemplary programming operation 1102, a first programming step 1110 b(analogous to operations 1012, 1014, and 1016 in FIG. 10) attempts toinitialize all of the bits within the array subsection (all originallyin unknown logic states represented by an “X”) into a RESET state (logic“0”). However, within this example, the third cell within the arraysubsection fails to initialize, introducing a RESET error 1115 b intothe subsection. In a next programming step 1120 b (analogous tooperations 1022 and 1024 in FIG. 10), the input data 1125 b (that is,the data due to be written into the subsection) is checked to see if theuninitialized cell is due to be programmed into a SET or RESET state.Since the third cell requires a logic “1” (that is, a SET state), theuninitialized cell does not require correction, as it is either alreadyin a valid and stable SET state (as required by input data 1125 b) orwill be further adjusted into such a state within programming step 1050b. As such, there is no need to invert the input data 1125 b as is donewithin the exemplary operation 1101 of FIG. 11A.

To this end, in this non-limiting example, the input data andinactivated flag bit are initially stored in a chip cache to facilitateBCH ECC encoding in process step 1150 b. Within programming step 1160 b(analogous to operation 1060 in FIG. 10), the BCH encoded input data isprogrammed into the array by adjusting the first cell and third cell ofthe subsection into a SET state, according to the input data 1125 b. Aspreviously discussed, since the third cell within the subsection isalready in a SET state (due to RESET error 1115 a), within certainapplications it may not be necessary to perform a SET operation on thiscell within programming step 1160 b. Within such operations, theuninitialized cell is already within a valid and stable SET state and nofurther adjustment to the cell is required. In other applications, a SEToperation may be performed on the third cell within programming step1160 b to ensure the cell is fully adjusted into a SET state. In eithercase, the SET condition stored within the third cell of the subsectionis correctly programmed according to the input data 1125 b despite RESETerror 1115 b being introduced during initialization.

In a next programming step 1170 b, the programmed data is read back outof the subsection and processed through a BCH ECC decoding algorithm(analogous to operation 1070 in FIG. 10). Since there are no othererrors within this example 1102, the data values within the subsectionremain unchanged. The flag bit was never activated (that is, was neverprogrammed to a logic “1” within this exemplary programming/read backoperation 1102) and there is no need to invert the subsection data, asis done within exemplary RDPM programming operation 1101 in FIG. 11A. Inthis way, the RESET error 1115 b introduced during the subsectioninitialization in programming step 1110 b is corrected.

Looking now to FIG. 11C, the single-flag DFEC method of the presentdisclosure is used within an exemplary programming operation 1103wherein a single programming error (SET error) is introduced on a datacell intended to be programmed into a SET state. At the start ofexemplary programming operation 1103, a first programming step 1110 c(analogous to operations 1012, 1014, and 1016 in FIG. 10) attempts toinitialize all of the bits within the array subsection (all originallyin unknown logic states represented by an “X”) into a RESET state (logic“0”). Unlike the exemplary programming operations 1101 and 1102 (withinFIGS. 11A and 11B, respectively), within this example all of the cellswithin the subsection initialize correctly and no RESET errors areintroduced. As such, there is no need to employ the single-flag DFECmethod of the present disclosure (that is, there is no need to activatethe subsection's flag bit or invert the input data 1125 c).

With the subsection initialized, within this non-limiting example, theinput data and inactivated flag bit are initially stored in a chip cacheto facilitate BCH ECC encoding in process step 1150 c. Withinprogramming step 1160 c (analogous to operation 1060 in FIG. 10), theBCH encoded input data is programmed into the array. As shown in FIG.11C, programming step 1160 c adjusts the first cell and third cell ofthe subsection into SET state, according to input data 1125 b. Withinexemplary programming operation 1103, a SET error 1145 c is introducedin the third cell of the subsection during programming step 1160 c. Thatis, the programming operation performed on the third cell of thesubsection—which should have adjusted the data cell from a RESET stateinto a SET state—fails during programming step 1160 c, and the thirddata cell within the subsection remains in a RESET state. As previouslydiscussed, the DFEC methods of the present disclosure are only able tocorrect initialization errors and, as such, cannot be used to correctSET error 1145 c. However, such an error can be corrected, according tothe methods of the present disclosure, by employing a second errorcorrection method in concert with the DFEC methods.

To this end, in a next programming step 1170 b, the programmed data isread back out of the subsection and processed through a BCH ECC decodingalgorithm (analogous to operation 1070 in FIG. 10). As shown in FIG.11C, this BCH ECC decoding corrects the SET error 1145 c introducedwithin programming step 1160 c, and the subsection is correctlyprogrammed according to input data 1125 c. Next, as the flag bit wasnever activated (that is, was never programmed to a logic “1” withinthis exemplary programming/read back operation 1103), there is no needto invert the subsection data as is done within exemplary RDPMprogramming operation 1101 in FIG. 11A. In this way, the SET error 1145c introduced within programming step 1160 c is corrected.

Looking now to FIG. 11D, the single-flag DFEC method of the presentdisclosure is used to correct two initialization errors (RESET errors)on two data cells, both intended to be programmed into a RESET state. Atthe start of exemplary programming operation 1104, a first programmingstep 1110 d (analogous to operations 1012, 1014, and 1016 in FIG. 10)attempts to initialize all of the bits within the array subsection (alloriginally in unknown logic states represented by an “X”) into a RESETstate (logic “0”). However, within this example, the second cell and thefourth cell within the array subsection both fail to initialize,introducing a first RESET error 1115 d and a second RESET error 1117 dinto the subsection. In a next programming step 1120 d (analogous tooperations 1022 and 1024 in FIG. 10), the input data 1125 d (that is,the data due to be written into the subsection) is checked to see if theuninitialized cells are due to be programmed into SET or RESET states.As both cells require a logic “0” (that is, a RESET state), bothuninitialized cells need to be corrected using the single-flag DFECmethod of the present disclosure.

To correct RESET errors 1115 d and 1117 d, programming step 1130 d(analogous to operation 1030 in FIG. 10) activates the subsection's flagbit, and DPM programming step 1140 d (analogous to operation 1040 inFIG. 10) inverts input data 1125 d. Within this non-limiting example,the inverted data and activated flag bit are initially stored in a chipcache to facilitate BCH ECC encoding in process step 1150 d. Withinprogramming step 1160 d (analogous to operation 1060 in FIG. 10), theBCH encoded inverted data is programmed into the array. As shown in FIG.11D, programming step 1160 d adjusts both the second cell and fourthcell of the subsection into a SET state, according to the inverted inputdata. As both the second and fourth cells within the subsection arealready in a SET state (due to RESET errors 1115 d and 1117 d), withincertain applications it may not be necessary to perform a SET operationon these cells within programming step 1160 d. Within such operations,the uninitialized cells are already within valid and stable SET statesand no further adjustment to the cells is required. In otherapplications, however, a SET operation may be performed on the first andthird cells within programming step 1160 d to ensure both cells arefully adjusted into a SET state. In either case, the SET conditionsstored within the first and third cells of the subsection are nowcorrectly programmed according to the inverted data.

In a next programming step 1170 d, the programmed data is read back outof the subsection and processed through a BCH ECC decoding algorithm(analogous to operation 1070 in FIG. 10). Since there are no othererrors within this example 1104, the data values within the subsectionremain unchanged. As the flag bit is activated (that is, programmed to alogic “1” within this exemplary programming/read back operation 1104),in RDPM programming step 1180 d (analogous to operations 1082 and 1084in FIG. 10) all of the bits within the subsection are inverted,correcting RESET errors 1115 d and 1117 d. In this way, the two RESETerrors 1115 d and 1117 d introduced during the subsection initializationin programming step 1110 d are both corrected using a single parity bit.

Looking now to FIG. 11E, the single-flag DFEC method of the presentdisclosure is used to correct two initialization errors (RESET errors)on two data cells: a first error on a data cell intended to beprogrammed into a RESET state, and a second error on a data cellintended to be programmed into a SET state. At the start of exemplaryprogramming operation 1105, a first programming step 1110 e (analogousto operations 1012, 1014, and 1016 in FIG. 10) attempts to initializeall of the bits within the array subsection (all originally in unknownlogic states represented by an “X”) into a RESET state (logic “0”).However, within this example, the first cell and the second cell withinthe array subsection both fail to initialize, introducing a first RESETerror 1115 e and a second RESET error 1117 e into the subsection. In anext programming step 1120 e (analogous to operations 1022 and 1024 inFIG. 10), the input data 1125 e (that is, the data due to be writteninto the subsection) is checked to see if the uninitialized cells aredue to be programmed into SET or RESET states.

As shown in FIG. 11E, the second cell within the subsection is due to beprogrammed into a RESET state (according to input data 1125 e) and thefirst cell within the subsection is due to be programmed into a SETstate (according to input data 1125 e). As will be shown withinexemplary programming operation 1105, the single-flag DFEC method of thepresent disclosure will be used to correct the second cell of thesubsection. However, this first operation will leave the first cell ofthe subsection in an erroneous state that will need to be corrected by asecond error correction operation (programming steps 1150 e and 1170 e).

As at least one of the uninitialized data cells requires a logic “0”(that is, a RESET state), programming step 1130 e (analogous tooperation 1030 in FIG. 10) activates the subsection's flag bit, and DPMprogramming step 1140 e (analogous to operation 1040 in FIG. 10) invertsinput data 1125 e. Within this non-limiting example, the inverted dataand activated flag bit are initially stored in a chip cache tofacilitate BCH ECC encoding in process step 1150 e. Within programmingstep 1160 e (analogous to operation 1060 in FIG. 10), the BCH encodedinverted data is programmed into the array. As shown in FIG. 11E,programming step 1160 e adjusts both the second cell and fourth cell ofthe subsection into a SET state, according to the inverted input data.As the second cell within the subsection is already in a SET state (dueto RESET error 1115 e), within certain applications it may not benecessary to perform a SET operation on this cell within programmingstep 1160 e. Within such operations, the uninitialized second data cellis already within a valid and stable SET state and no further adjustmentto the cell is required. In other applications, however, a SET operationmay be performed on the second cell within programming step 1160 e toensure the cell is fully adjusted into a SET state. In either case, theSET condition stored within the second cell of the subsection is nowcorrectly programmed according to the inverted data 1145 e.

In a next programming step 1170 e, the programmed data is read back outof the subsection and processed through a BCH ECC decoding algorithm(analogous to operation 1070 in FIG. 10). As shown in FIG. 11E, this BCHECC decoding corrects RESET error 1115 e (introduced within programmingstep 1110 e). Next, as the flag bit is activated (that is, programmed toa logic “1” within this exemplary programming/read back operation 1105),in RDPM programming step 1180 e (analogous to operations 1082 and 1084in FIG. 10) all of the bits within the subsection are inverted,correcting RESET error 1117 e. In this way, the two RESET errors 1115 eand 1117 e introduced during the subsection initialization inprogramming step 1110 e are both corrected using the single-flag DFECmethod of the present disclosure in concert with a second errorcorrection routine (the BCH error correction cycle of programming steps1150 e and 1170 e).

FIG. 12A is a simplified schematic diagram illustrating an exemplaryencoding circuit 1201 suitable for use with the RFEC method of thepresent disclosure. As with other examples within the presentdisclosure, exemplary encoding circuit 1201 is arranged for use with aresistive change element array that uses RESET (that is, logic “0”) asits initialization state. However, it should be noted that, aspreviously discussed, the methods of the present disclosure are notlimited in this regard. The DFEC methods of the present disclosure canalso be used within a resistive change element array that uses a SETstate as an initialization condition, and exemplary encoding circuit1201 is intended only as a non-limiting, illustrative example of acircuit capable of encoding programming data according to thesingle-flag DFEC method of the present disclosure.

Looking now to FIG. 12A, an array of AND gates 1225 ANDs together thedata cells of a resistive change element array subsection 1210 withinput data 1215 (analogous to input data 1025 in FIG. 10) which isinverted through an array of Inverter gates 1220. The outputs of ANDgates 1225 are ORed together with OR gate 1230 to provide the flag bitfor the subsection 1245. Such a configuration will activate (that is,set to a logic “1” within this exemplary encoding circuit 1201) flag bit1245 if any data cell within subsection 1210 fails to initialize (thatis, if any data cell is in a SET state) and is intended to be programmedinto a RESET state. Such a logic function is analogous to operations1014, 1016, 1022, 1024, and 1030 within FIG. 10, discussed in detailwith respect to that figure above.

Next, an array of Exclusive OR gates 1235 selectively inverts the inputdata 1215 dependent on the state of flag bit 1245. The array ofExclusive OR gates 1235 then provides either the input data 1215 (ifflag bit 1245 is not activated) or an inverted version of the input data1215 (if flag bit 1245 is activated) to be programmed into the array(the program data represented by block 1240 in FIG. 12A). Such a logicfunction is analogous to operation 1040 within FIG. 10, discussed indetail above.

FIG. 12B is a simplified schematic diagram illustrating an exemplarydecoding circuit 1202 suitable for use with the single-flag DFEC methodof the present disclosure. As with encoding circuit 1201 within FIG. 12Aas well as other examples within the present disclosure, exemplarydecoding circuit 1202 is arranged for use with a resistive changeelement array that uses RESET (that is, logic “0”) as its initializationstate. Again, it should be noted that, as previously discussed, themethods of the present disclosure are not limited in this regard. TheDFEC methods of the present disclosure can also be used within aresistive change element array that uses a SET state as aninitialization condition, and exemplary decoding circuit 1202 isintended only as a non-limiting, illustrative example of a circuitcapable of decoding program data according to the single-flag DFECmethod of the present disclosure.

Looking now to FIG. 12B, an array of Exclusive OR gates 1280 selectivelyinverts read data 1260 (that is, data read out of an array subsection)dependent on the state of flag bit 1270. The array of Exclusive OR gates1280 then provides either the read data 1260 (if flag bit 1270 is notactivated) or an inverted version of the read data 1260 (if flag bit1270 is activated) to the output of the resistive change element array1290. Such a logic function is analogous to operations 1082 and 1084within FIG. 10, discussed in detail with respect to that figure above.

As shown within exemplary programming operations 1101-1105 within FIGS.11A-11E, the methods of the present disclosure employ the RFEC method ofthe present disclosure in concert with a second error correction routineto provide error correction within a resistive change element array at adesired correctable bit error rate (BER) with reduced parity overhead ascompared to using a conventional error correction routine alone.

Statistical Analysis of the DFEC Methods

As previously discussed, flag bits distributed across a resistive changeelement array (or, block) can delimitate DFEC subsections, wherein thedistributed flag bits may be used to correct at least one initializationerror per subsection in a first operation, and thereby reduce the numberof errors required to be corrected by a conventional error correctionroutine performed within a second operation to achieve a desiredcorrectable BER. The number of flag bits employed may substantiallyaffect the number of errors which may be corrected by the DFEC methods.Further, by selecting the size of the subsections within the resistivechange element array, the overall parity overhead (that is, the totalnumber of DFEC flag bits plus the reduced number of parity bits used forthe conventional error correction routine) can be selectively reduced asbefits the needs of a particular application. Thus, as will be discussedin FIGS. 13A-13D, 14A-14D, 15A-15D, and 16A-16C, for a given correctableBER (which may be determined experimentally for a particular resistivechange element array design), the number of flags and the size of thesubsections may be selected to optimize the performance and minimize theparity overhead of a resistive change element array, without reducingthe effectiveness of the error correction routine.

In addition to the improvements in parity overhead (as discussed above),the reduced number of parity cells realized using the DFEC methods ofthe present disclosure can also reduce decoding latency with an errorcorrection operation. For example, BCH ECC decoding latency, t_(ECC), isgiven by:

t _(ECC)=(m*t/p+(t+1)f/2+t−1+n/p)/F  EQ3

Within EQ3, m is the number of required parity bits to recover one biterror (as given by EQ1), n is BCH ECC code length (that is, the numberof data bits, plus the number of flag bits, plus the number of paritybits for secondary error correction scheme which equals tom*t when BCHis used), p is the number of decoding circuits to work in parallel, f isthe circuit folding factor, F is circuit operating frequency in MHz, andt is the number of BCH ECC correctable bits. Using EQ3, an exemplary512-byte resistive change element array wherein m=13 (as given by EQ1),t=8 (corresponding to a 0.2% correctable BER), p=32, f=12, F=200, andn=4200 (4096 data bits+104 parity bits) would have a BCH decodinglatency of 0.98 μs. However, as discussed above, using the DFEC methodsof the present disclosure, the number of BCH ECC correctable bits “t”can be significantly reduced for a required correctable BER. Forexample, if the single-flag DFEC method of the present disclosure can beexpected to correct four errors within this exemplary array, “t” withinEQ3 can be reduced to 4, resulting in a BCH ECC decoding latency of 0.82μs. The decoding latency for the single-flag DFEC method of the presentdisclosure can be ignored because the method can be implemented using arelatively simple decoding circuit (as discussed with respect to FIG.12B above). As such, within this example, the RFEC method of the presentdisclosure provides a 16% improvement in total ECC decoding latency.Such an improvement in performance can be highly desirable withincertain applications such as, but not limited to, solid state harddrives and non-volatile memory applications.

To this end, FIGS. 13A-13D, 14A-14D, 15A-15D and 16A-16C illustrate theeffects of subsection size selection and correctable BER on the parityoverhead and ECC decoding latency within an exemplary resistive elementchange array for three DFEC methods. FIGS. 13A-13D are directed at theoptimization of the single-flag DFEC (RFEC) method of the presentdisclosure by varying subsection size, while FIGS. 14A-14D and 15A-15Ddemonstrate this optimization with the dual reverse flag (DRFEC) andthree-flag 2-bit advanced bit flip (ABF) methods, respectively. FIGS.16A-16C provide a comparison of these three DFEC methods of the presentdisclosure for different correctable BERs and subsection sizes. Itshould be noted, however, that the statistical analysis presented inFIGS. 13A-13D, 14A-14D, 15A-15D and 16A-16C is for illustrative purposesonly in order to highlight the ways in which optimization of the DFECmethods for specific applications may be accomplished. As such, the datapresented in FIGS. 13A-13D, 14A-14D, 15A-15D and 16A-16C is exemplaryand in no way should be taken to reflect experimental data. Theconclusions drawn therefrom may differ according to the properties andperformance of a particular resistive change element array and theengineering requirements of real applications.

The exemplary resistive change element array used in FIGS. 13A-3D,14A-14D, 15A-15D and 16A-16C is an 8 kilobyte array arranged into eight1 kilobyte blocks of user data, analogous to array 901 in FIG. 9A, array1701 of FIG. 17A, or array 2201 of FIG. 22A, depending on the DFECmethod employed. The error distribution within this exemplary resistivechange element array is assumed to be 75% RESET and 25% SET, asdescribed previously in FIGS. 7A-7D, unless otherwise noted. As such,the initialization state of the array is taken to be RESET according tothe state which is expected to result in a higher proportion of errors,though the invention is not limited in the initialization state chosen.

FIG. 13A is a table 1301 detailing the parity overhead for the exemplary8 kilobyte resistive change element array with five different subsectionsizes, wherein a correctable BER of 0.2% is required. As such, acorrectable BER of 0.2% would require an error correction routine to becapable of correcting 16 errors per block (16 errors divided by 8192bits/block+parity bits/block yielding about 0.2%). Since the exemplaryresistive change element array of FIG. 13A can be expected to yield 75%RESET errors and 25% SET errors, a suitable error correction routine forsuch an array will be required to correct 12 RESET errors and 4 SETerrors within each one kilobyte (8192 bit) block of user data. Withinthe exemplary array of FIG. 13A, the RFEC method of the presentdisclosure is used in a first operation (requiring one flag bit persubsection), and a BCH error correction cycle (requiring parity bitsaccording to EQ. 1 as described above) is used in a second operation (asis described within FIG. 10 and illustrated in FIGS. 11A-11E) to realizea correctable BER of 0.2%.

Looking again to FIG. 13A, each row of table 1301 demonstrates a singleblock of this exemplary resistive change element array with one kilobyteof user data divided into a different number of subsections (4, 8, 16,32, and 64 subsections within the first through fifth rows of table1301, respectively). For each subsection configuration, the flag bitparity overhead, the BCH parity bit overhead, and the total parityoverhead are calculated using EQ1 and EQ2 as described in detail above.

Within the first row of table 1301, each one kilobyte data block withinthe exemplary resistive change element array is divided into four (4)subsections, with each subsection containing 256 bytes (1024/4=256) or2048 bits. Ideally, the single-flag DFEC method of the presentdisclosure can be expected to correct at least one RESET error persubsection (RESET being the initialization condition within theexemplary array). However, as defined above, within the exemplaryresistive change element array of FIG. 13A it is assumed that RESETerrors will occur only 75% of the time. Further, due to the statisticaldistribution of these RESET errors (that is, some subsections maycontain no errors, or multiple errors), the four distributed flag bitswithin each block (one flag bit per subsection) will be statisticallycapable of correcting only three (3) RESET errors per block, asindicated by the fourth column of table 1401, which is less than theideal case wherein each flag bit corrects at least a single error. Thus,as can be seen in rows three through five, due to the error distributionthe DFEC methods may not be expected to correct all of the errors in thearray. With each of the subsections including one flag bit and 2048 databits, this configuration yields a flag bit parity overhead of 0.05%(using EQ2: 1 flag bit/2048 data bits=0.05%).

Looking now to the sixth column of table 1301 (BCH Correctable Errors),as the single-flag DFEC method of the present disclosure can be expectedto correct three (3) RESET errors within a first operation, a BCH errorcorrection routine used in a second operation only needs to address theremaining nine (9) RESET errors and the four (4) SET errors to realize a0.2% correctable BER, as described above. As such, the BCH errorcorrection cycle only needs to correct 13 errors total. Using EQ1 asdescribed above, it can be seen that a BCH error correction cyclerequires 14 BCH parity bits to correct a single error within a 1kilobyte block of user data. As such, correcting the remaining 13 errorswith a BCH routine requires 182 BCH parity bits (14 BCH parity bits×13errors), as listed in the seventh column of table 1301. Using EQ2, thisyields a BCH parity overhead for the entire block of 2.2% (182 BCHparity bits/8192 data bits) and a total parity overhead for the block of2.3% (186 total parity bits/8192 data bits), as shown in the eighth andninth columns of table 1301, respectively. This parity overhead can becompared to an error correction scheme using BCH alone, wherein the BCHerror correction cycle would be required to correct all 16 errors. UsingEQ1, such a system using only BCH ECC would require 224 BCH parity bits(14 BCH parity bits×16 errors), yielding a total parity overhead (fromEQ2) of 2.7% (224 BCH parity bits/8192 data bits). Looking to the tenthcolumn of table 1301, the decoding latency for the 182 BCH parity bits(as given by EQ3, discussed in detail above) is 1.18 μs for this foursubsection configuration. This can be compared to an ECC decodinglatency of 1.30 μs for a BCH ECC only system, which would require 224BCH parity bits, as described above. As such, the use of the RFEC methodof the present disclosure provides a significant reduction in parityoverhead and ECC decoding latency as compared with using a BCH errorcorrection cycle alone.

The next four rows of table 1301 repeat the above calculations for arrayblocks that are divided into 8, 16, 32, and 64 subsections,respectively. As can be seen from the second row of table 1301, eight128-byte subsections are capable of correcting 5 RESET errors in an8-kilobyte block, leaving only 7 RESET errors and 4 SET errors for theBCH error correction cycle and yielding a total parity overhead of 2.0%and a decoding latency of 1.10 μs. Parity overhead is further improved,as shown in the third row of table 1301, by using sixteen 64 bytesubsections. Within such a configuration, the DFEC flag bits can beexpected to correct 8 RESET errors, leaving only 4 RESET and 4 SETerrors for a BCH error correction cycle and yielding a total parityoverhead of 1.6% and a decoding latency of 0.98 μs.

Looking to the fourth row of table 1301, using thirty-two 32-bytesubsections provides 32 DFEC flag bits within each block, which areassumed to correct 9 RESET errors. However, despite this increase ascompared with a sixteen subsection block, the reduction in BCH paritybits is compensated by an increased number of flag bits. As such, theparity overhead required to realize 0.2% correctable BER with such aconfiguration remains at 1.6%, nearly the same as for the sixteensubsection configuration, and the ECC decoding latency is slightlyimproved to 0.94 μs. Looking to the fifth row of table 1301, it is shown(using the same calculations) that a configuration of sixty-four 16-bytesubsections further increases total parity overhead to 1.8%, with theECC decoding latency reducing further to 0.9 μs.

FIG. 13B is a bar graph 1302 plotting the parity overhead calculationsfor each configuration within FIG. 13A compared with the parity overheadof a BCH ECC only configuration (2.7%, as described in detail above). Asshown within the bar graph plot 1302, for the exemplary resistive changeelement array of FIG. 13A (as defined in detail within the discussion ofthat figure), the DFEC configuration that provides the lowest overallparity overhead is a configuration using 16 flag bits to delimitatesixteen 64-byte subsections per block. As described above, such aconfiguration would use 16 flag bits (1 flag bit for each subsection) tocorrect 8 RESET errors and 112 BCH parity bits to correct 4 RESET and 4SET errors, yielding a total parity overhead of 1.6%.

Similarly, FIG. 13C is a table 1303 detailing the parity overhead for anexemplary resistive change element array over five different subsectionsizes wherein a correctable BER of 0.3% is required. The exemplaryresistive change element array used within the calculations of table1303 is identical to the array used within FIGS. 13A and 13B, however,in this case RESET errors are expected to occur 83% of the time (for 24total errors, 20 are expected to be RESET and 4 are expected to be SETerrors). For such a resistive change element array, a correctable BER of0.3% requires an error correction routine capable of correcting 24errors (20 RESET errors and 4 SET errors) within a 1-kilobyte block ofuser data.

Using the same calculations as discussed with respect to FIG. 13A, thefirst row of table 1303 shows that for an array configuration of four256-byte subsections per 1 kilobyte block, the single-flag DFEC methodof the present disclosure can be expected to provide a total parityoverhead of 3.6% with a decoding latency of 1.33 μs for a correctableBER of 0.3% (as compared to a 4.1% parity overhead with a decodinglatency of 1.61 μs for a BCH only system). Similar calculations for thenext four rows show total parity overheads of 3.2%, 2.4%, 2.1%, and 2.0%and ECC decoding latencies of 1.14 μs, 0.98 μs, 0.90 μs, and 0.86 μs forconfigurations of 128 bytes per subsection, 64 bytes per subsection, 32bytes per subsection, and 16 bytes per subsection, respectively. As withthe exemplary configurations of FIGS. 13A and 13B, within theconfigurations of FIG. 13C, a suitable subsection size for a givenapplication is determined by plotting the results of these calculationswithin the bar graph plot 1304 of FIG. 13D.

Looking now to FIG. 13D, it is shown that for a 0.3% correctable BER,the exemplary resistive change element array of FIG. 13C would use thelowest total parity overhead using a configuration of sixty-four 16-bytesubsections per block. Such a configuration would use 64 flag bits (1flag bit for each subsection) to correct 9 RESET errors and 98 BCHparity bits to correct 3 RESET and 4 SET errors, yielding a total parityoverhead of 2.0%. As with the results of FIGS. 13A and 13B, this parityoverhead can be compared to an error correction scheme using BCH alone,wherein the BCH error correction cycle would be required to correct all24 errors. Using EQ1 again, such a system using BCH ECC alone wouldrequire 336 BCH parity bits (14 BCH parity bits×24 errors), yielding atotal parity overhead (from EQ2) of 4.1% (336 BCH parity bits/8192 databits). As such, the use of the single-flag DFEC method of the presentdisclosure provides a significant reduction in parity overhead ascompared with using BCH error correction cycle alone.

It should again be noted that the selection of a subsection size asdetailed in FIGS. 13A-13D and further examples below is dependent on thespecific parameters of a resistive change element array such as aredescribed in detail with respect to those figures above. Such parametersinclude, but are not limited to, the size of the resistive changeelement array, the required correctable BER, the expected error rate,the distribution and type of errors within the array, and the type andconfiguration of the secondary error correction routine used inconjunction with the DFEC methods of the present disclosure. Thecalculations described above and the results listed and plotted withinFIGS. 13A-13D are intended as non-limiting, illustrative examples. For agiven application, careful consideration of such parameters and thespecific needs of the application can be used to select the mostsuitable array configuration (as described above) for use with the DFECmethods of the present disclosure.

It should also be noted that the combination of the DFEC methods of thepresent disclosure with a second error correction method (such as a BCHECC method, as discussed throughout the present specification) can beuseful as the second error correction method can be used to recover dataretention errors as well as programming errors. As previously discussed,the DFEC methods of the present disclosure can only be used to correctinitialization errors introduced during a programming operation. Withincertain applications, other types of errors (such as, but not limitedto, data retention errors) can also be present within a resistive changeelement array. As such, the use of the DFEC methods of the presentdisclosure in concert with a second error correction scheme can behighly desirable within such applications.

FIGS. 14A-14D repeat the above calculations for the dual reverse flagerror correction (DRFEC) DFEC method, in which two flags are used toindicate the presence of errors on an even or an odd data bit. Asdiscussed previously, the exemplary resistive change element array ofFIGS. 14A-14D is identical to that of FIGS. 13A-13D, except that it hasbeen configured to use two flag bits per subsection rather than one. Theadditional flag bit may allow a greater percentage of errors to becorrected by the DRFEC method, particularly continuous errors (errorsoccurring on two adjacent data bits).

FIG. 14A is a table 1401 detailing the necessary parity overhead toachieve a 0.2% correctable BER in a single resistive change elementblock at different subsection sizes, using the DRFEC method of thepresent disclosure. Theoretically, each DRFEC flag may be expected tocorrect at least one odd and at least one even error in a subsection, animprovement over the single-flag DFEC method described above. However,as shown in the third column of 1401 and discussed previously, a 0.2%BER on a 1-kilobyte block results in 16 errors per block—12 RESET and 4SET errors—which occur randomly across the array. Larger subsections,such as the first two rows of table 1401, are therefore likely tocontain multiple errors which cannot all be corrected, while smallsubsections may contain zero errors due to the statistical nature oferror distribution. Thus, as can be seen in the fourth column of table1401, the four subsection (8 flag) configuration may only be expected tocorrect 5 errors, the eight subsection (16 flag) configuration may onlycorrect 8 errors, the sixteen subsection configuration may only be ableto correct 9 errors, and so on for the smaller subsections.

The random nature of error occurrence, therefore, may requireoptimization of the subsection size to achieve a given correctable BERwith greatest efficiency. The sixth column of table 1401 details theerrors remaining after use of the DFEC methods which must be correctedby BCH ECC. As can be seen in the first row, for instance, BCH mustcorrect 7 RESET and 4 SET errors. From EQ1 above, BCH requires 154parity bits (14 parity bits/error×11 errors) to correct all remainingerrors in the array, yielding a total parity overhead of 2.0%, as seenin the ninth column, which includes both BCH parity bits and eight (8)DFEC flag bits. This may be compared to the 224 BCH parity bits and 2.7%parity overhead required to correct all 16 errors using BCH ECC alone.Looking now at the eight and sixteen subsection configurations, it maybe seen that smaller subsections result in fewer required BCH paritybits but also result in higher flag bit overhead. Thus, the overallparity overhead remains nearly constant between these two subsectionsdespite reducing the number of BCH parity bits. Reducing the subsectionsize further continues to reduce the BCH parity overhead, but thisreduction is outweighed by the increasing flag bit overhead. Indeed, atsixty-four subsections, the overall parity overhead of 2.4% nearlyreaches the overhead required for BCH alone, 2.7%. That said, however,reducing the number of BCH parity bits may also improve decoding latencyaccording to EQ3, as shown in the tenth column of 1401. Using foursubsections, for instance, may reduce the decoding latency from 1.3 μsfor BCH alone to 1.1 μs for the DRFEC method, while using thirty-twosubsections may reduce the latency further to 0.9 μs. Thus, smallersubsections, such as those of the thirty-two subsection configuration,may reduce the decoding latency despite increasing the overall parityoverhead. As will be shown in at least FIG. 14B, these tradeoffs may beused to optimize the DFEC configuration employed for a specificapplication. In this way, the DRFEC method of the present disclosure maybe used to improve the efficiency of error correction in a resistivechange element array.

FIG. 14B depicts the overall parity overhead data displayed in table1401 graphically in chart 1402. As explained previously, using BCH ECCalone would require 224 parity bits (14 parity bits/error×16 errors) tocorrect all of the errors, yielding a parity overhead of 2.7%. As can beseen, all of the DRFEC subsection configurations provide reduced parityoverhead compared to BCH ECC alone. The eight and sixteen subsectionconfigurations, however, have nearly identical parity overhead, at 1.6%.In some applications, therefore, since 1.6% also happens to be thesmallest parity overhead in these examples, the sixteen subsectionconfiguration may be ideal over the eight subsection configuration, asit possesses lower decoding latency due to the smaller number of BCHparity bits. In this way, the DRFEC configuration may be selectedaccording to the requirements of a specific application.

FIG. 14C is a table 1403 detailing the necessary parity overhead toachieve a 0.3% correctable BER in a single resistive change elementblock at different subsection sizes, using the DRFEC method of thepresent disclosure. In this case, however, the RESET error proportion inthe array is slightly higher, at 83%. Thus, for 24 total errors (0.3% of8192 data bits), 20 RESET and 4 SET errors may be expected. As describedwith respect to FIGS. 14A and 14B above, the DFEC flag bits may not beable to correct as many errors as their theoretical maximum due to thestatistical nature of the error distribution. Even so, as can be seen intable 1403, the DRFEC method may be used to dramatically reduce thenumber of BCH parity bits. The sixteen subsection configuration, forinstance, can be expected to correct 14 errors, thereby reducing thenumber of BCH parity bits from 336 (14 parity bits/error×24 errors) forBCH alone to 140, and the total parity overhead from 4.1% to 2.1%—nearlya 50% reduction.

In order to determine the optimal configuration for these conditions,FIG. 14D plots the data of table 1403 in chart 1404. Here, the DRFECmethod may be seen to dramatically reduce the parity overhead comparedto BCH alone. In this case, the thirty-two subsection configuration maybe seen to have the smallest parity overhead—2.0%. Recalling theprevious examples of FIG. 14A and 14B at 0.2% BER, in which the optimalconfiguration was a sixteen subsection block, it is clear that a higherBER requires smaller subsections to separate the errors into separatesubsections. This demonstrates the ability to optimize the DFECconfiguration based on the expected correctable BER and the requirementsof specific applications.

FIGS. 15A-15D repeat the above exemplary statistical analysis for thethree-flag 2-bit advanced bit flip (ABF) DFEC method of the presentdisclosure, wherein three flag bits are used to store a patternreference code (PRC) corresponding to an encoding pattern which encodesa set of input data which has been divided into 2-bit subsets. Asmentioned previously, the exemplary resistive change element array usedin these examples is identical to those of FIGS. 13A-13D and 14A-14Dabove, except that it has been configured with three DFEC flag bits persubsection. Again, it should be noted that the statistical analysispresented in FIGS. 15A-15D, as well as FIGS. 13A-13D and 14A-14D above,is for illustrative purposes only, in order to highlight the ways inwhich optimization of the DFEC methods for specific applications may beaccomplished. As such, the data presented in FIGS. 15A-15D, and in FIGS.13A-13D and 14A-14D above, is exemplary and in no way should be taken toreflect experimental data. The conclusions drawn therefrom may differaccording to the properties and performance of a particular resistivechange element array and the engineering requirements of realapplications.

Looking now to FIG. 15A, table 1501 details the required BCH parity bitsand total parity overhead for five subsection sizes using the three-flag2-bit ABF method of the present disclosure. As described previously andin more detail below, this ABF method is guaranteed to correct at leasttwo initialization errors per subsection (if two errors in fact occur),leading to a substantially improved error correction rate. This may beseen in the fourth column, wherein the four subsection method maycorrect 6 RESET errors (compared to the RFEC and DRFEC methods, whichmay correct only 3 and 5 RESET errors, respectively). Hence, at sixteensubsections, the total parity overhead is reduced to 1.4%, compared tothe 2.7% overhead required for BCH alone. It can be seen, however, thatat sixteen subsections essentially all RESET errors are corrected bythis ABF method alone—11 of the total 12 RESET errors—and that smallersubsections do not improve the number of errors corrected. Accordingly,the number of BCH parity bits reaches a minimum at 70 parity bits forthe sixteen subsection configuration, and smaller subsections increasethe number of flag bits substantially without affecting the BCH parityoverhead, leading to an increase in overall parity overhead. Indeed, atsixty-four subsections the overall parity overhead is 3.1%, which iseven larger than the parity overhead for BCH alone (2.7%).

Thus, as shown in chart 1502 of FIG. 15B, the three-flag ABF method ismost efficient at sixteen subsections. Increasing the number ofsubsections further simply increases the flag bit overhead withoutreducing the number of BCH parity bits. This behavior is similar to thatshown in FIGS. 13B and 14B, but is exacerbated by the larger number offlag bits per subsection. That said, as will be discussed in detail withrespect to FIGS. 16A-16C, this ABF method is clearly the most effectiveof the DFEC methods considered at correcting errors, despitesignificantly reduced efficiency at smaller subsections. This highlightsthe ability to tailor the DFEC methods to specific applications, both interms of the DFEC method employed (RFEC, DRFEC, or ABF), and in thespecific configurations thereof.

FIG. 15C continues this analysis of the three-flag 2-bit ABF method fora correctable BER of 0.3%, detailed in table 1503. As with FIGS. 13C and14C, the proportion of RESET errors in this case is taken to be 83% (20RESET errors per 24 total errors) rather than 75%, as in FIG. 15A. Thehigher number of errors, and higher proportion of RESET errors, isexpected to impact the optimization of this ABF method by increasing thelikelihood of multiple errors in a single subsection. Thus, it may beseen that, unlike in FIG. 15A, the number of errors corrected by thisABF method does not reach a maximum in the examples provided. That said,however, the lowest total parity overhead still occurs at sixteensubsections—2.0%, compared to the 4.1% required for BCH alone—while thenumber of BCH parity bits and decoding latency continues to decreasewith smaller subsections.

FIG. 15D depicts these results graphically in chart 1504. Here it may beseen that between four and sixteen subsections the total parity overheaddecreases rapidly. Beyond this, the increase in flag bit parity overheadoutweighs the decrease in BCH parity overhead, but unlike FIG. 15B stillprovides a reduced parity overhead when compared to BCH alone. Thesixteen subsection configuration may therefore be considered the idealconfiguration in certain applications. In other applications, however,the reduced decoding latency at thirty-two subsections (as a result ofthe smaller number of BCH parity bits) may be more important than thesmall increase in parity overhead over the sixteen subsectionconfiguration—2.2% compared to 2.0%. Thus, this ABF method may be moreefficient at high correctable BER due to its higher effectiveness aterror correction, and may be optimized according to the needs of aspecific application.

These trends may be seen clearly in the comparison of these three DFECmethods depicted graphically in FIGS. 16A-16C. FIG. 16A is a chart 1601comparing the total parity overhead for the RFEC (single-flag), DRFEC(dual-flag) and three-flag 2-bit ABF methods to BCH alone for differentcorrectable bit error rates. In this example a thirty-two subsectionconfiguration is used, giving a total of 32, 64 and 96 flag bits perblock for the three DFEC methods, respectively. Further, in thisexample, the error distribution is taken to be 100% RESET. That is, fora 0.2% BER, a RESET error rate of 0.2% is expected, unlike the previousexamples in which a 75% RESET error rate is assumed, wherein a BER of0.2% would result in a RESET error rate of 0.15%. Thus, compared toprevious examples at 0.2% BER, the total number of errors is the samebut the number of RESET errors has increased. As indicated by thearrows, at the lowest BER of 0.1%, the RFEC method is most efficient,since few errors are expected and a single flag is sufficient. At 0.2%BER, both the RFEC and DRFEC methods provide comparable parity overhead(about 1.1%). In this case, although the RFEC method provides slightlylower overhead, the DRFEC method significantly reduces the number of BCHparity bits, resulting in lower decoding latency (0.78 μs compared to0.82 μs). Depending on the needs of a particular application, eithermethod may be considered advantageous.

At higher BER (above 0.2%), the three-flag ABF method proves moreefficient in both parity overhead and decoding latency. As explainedpreviously, this is a result of the higher effectiveness of this ABFmethod at correcting RESET errors despite requiring a larger number offlag bits. Indeed, at a correctable BER of 0.5%, this ABF method mayrequire a parity overhead of only 2.4% compared to 6.8% for BCH alone.This implies a reduction of nearly 65%. In this way, the DFEC methods ofthe present disclosure may be used to dramatically reduce the number ofBCH parity bits and overall parity overhead required to achieve a givencorrectable BER.

Next, FIG. 16B depicts a chart 1602 comparing the parity overheadrequired by the RFEC, DRFEC, and the three-flag 2-bit ABF methods of thepresent disclosure to achieve a correctable BER of 0.2% in fivesubsection configurations, using the data previously presented in FIGS.13A, 14A, and 15A. As described previously, and indicated by the arrowsin chart 1602, it may be seen that this ABF method is most effective forlarger subsections due to its higher effectiveness at RESET errorcorrection. For smaller subsections, such as the thirty-two andsixty-four subsection configurations, the RFEC method proves mostefficient. Again, this is due to this ABF method requiring only sixteensubsections to correct substantially all of the RESET errors, andincreasing the number of subsections further simply increases the flagbit overhead without affecting the number of BCH parity bits required.Indeed, for a sixty-four subsection configuration, the ABF methodactually increases the overall parity overhead compared to BCH alone.

Accordingly, as demonstrated in FIG. 16C, the three-flag 2-bit ABFmethod may be more effective at higher correctable BER. Here, chart 1603shows that this ABF methods is again the most efficient of the DFECmethods considered for large subsections, up to the sixteen subsectionconfiguration. At thirty-two subsections, however, the DRFEC methodprovides the lowest parity overhead, and at sixty-four the RFEC methodis most efficient. This demonstrates the ability to tailor the DFECmethods to address the needs of a specific application by optimizing theDFEC method employed, and the specific configuration thereof. Further,FIGS. 13A-13D, 14A-14D, 15A-15D, and 16A-16C show how the DFEC methodsof the present disclosure may be used to substantially reduce the totalparity overhead and decoding latency required to achieve a givencorrectable BER for a resistive change element array.

Dual Reverse Flag Error Correction

As described previously with respect to FIGS. 9A-9C, 10, 11A-11E,12A-12B and 13A-13B, the single-flag DFEC method may be used to correctat least a single initialization error per subsection in a resistivechange element array. This method, however, provides only a 50% chanceof correcting two initialization errors per subsection. Thus, in certainapplications wherein the error distribution is expected to result inmultiple errors per subsection, or a large degree of continuous errors(that is, errors occurring on two adjacent data bits), the single-flagDFEC method may be insufficient. In these circumstances, two flag bitsper subsection may be employed to increase the error correction rate.

FIGS. 17A-17B, 18, and 19A-19F teach a second DFEC method using dualflag bits. In this dual reverse flag error correction (DRFEC) DFECmethod, two flag bits per subsection are distributed across a resistivechange element array to delimitate each subsection. Similarly to theRFEC method, these two flag bits are used to indicate the presence ofinitialization errors within the subsection and to invert the input dataaccordingly. However, in this case, one flag bit indicates errors oneven data bits while the other indicates errors in the odd data bits.Thus, the input data may be matched to the error pattern in four ways—noeffective errors (neither flag bit is activated), at least one odd errorwith no effective even errors (only the odd flag bit is activated), atleast one even error with no effective odd errors (only the even flagbit is activated), or at least one even and at least one odd error (bothflag bits are activated), as will be described in more detail duringdiscussion of FIGS. 18 and 19A-19F. In this way, the dual-flag DFECmethod may be guaranteed to correct at least one even and one odd errorper subsection.

FIG. 17A depicts an exemplary resistive change element array 1701configured for use with the DRFEC method of the present disclosure inconcert with a secondary error correction method, in this case BCH.Resistive change element array 1701 comprises multiple, in this example,eight, BCH ECC memory blocks 1710, which are illustrated in more detailin FIG. 17B. Each memory block 1710 is divided into several, in thisexample, four, DFEC subsections 1711, which comprise data bits 1711 aand two DFEC flag bits 1711 b. Each memory block 1710 additionallycomprises BCH parity bits 1712. Initialization errors may occurthroughout resistive change element array 1701, including data biterrors 1720 which occur in data subsections 1711 a, BCH parity errors1730 which occur in the BCH parity bits 1712, and DFEC flag errors 1740which occur in DFEC flag bits 1711 b. As described previously withrespect to FIGS. 9A-9C, each of the DFEC flag bits 1711 b in subsections1711 are capable of correcting at least one data bit initializationerror 1720 per subsection 1711. DFEC flag bit initialization errors1740, BCH parity bit errors 1730, and all other errors must be correctedby a secondary error correction method. The use of the dual-flag DFECmethod, however, may substantially reduce the number of errors requiredto be corrected by this secondary error correction method for a givenBER. Since the secondary error correction methods require many paritybits 1712 to correct a single error (BCH ECC requires 14 paritybits/error for an 8 kilobyte block, for instance), compared to DFECmethods of the present disclosure which require only a single flag bitto correct a single data bit initialization error 1720, the use of theDFEC methods of the present disclosure may substantially reduce theparity overhead for a given correctable BER.

FIG. 17B depicts the exemplary memory block 1710 of FIG. 17A in moredetail. As can be seen, memory block 1710 comprises multiple, in thiscase, four, subsections 1711, as well as parity bits 1712 for use with aselected secondary error correction method (such as, but not limited to,BCH). Subsections 1711 comprise data bits 1711 a and DFEC flag bits 1711b. In this example, memory block 1710 is configured for the DRFEC methodand hence DFEC flag bits 1711 b comprise two flag bits 1715 a and 1715b, which may be designated even or odd according to the needs of acertain application. Thus, flag bit 1715 a may indicate even errorswithin data section 1711 b while flag bit 1715 b indicates the odderrors, or flag bit 1715 a may indicate odd errors while flag bit 1715 bindicates the even errors, the invention is not limited in this regard.

It should be noted that in the present disclosure bits are designatedeven or odd according to their position within the array in analternating pattern. Thus, if the first bit in a data section isconsidered even, the second will be odd, the third even, and so on in analternating pattern to the end of the data section. Alternatively, ifthe first bit in a data section is considered odd, the second will beeven, the third odd, and so on in an alternating pattern to the end ofthe data section. For ease of explanation, the present disclosure refersto the first data bit in a data section as even, but the invention isnot limited in this regard.

FIG. 18 is a flowchart depicting the use of the dual-flag DFEC method ofthe present disclosure during a programming/read operation 1800 on asingle subsection of a resistive change element array, such as thatdepicted in FIG. 17A. In this example, the initialization state of thearray is taken to be RESET (or, logic “0”), but the invention is notlimited in this regard. In a first process step 1812 the subsection isinitialized, wherein all cells (including both DFEC flags and datacells) are placed into a RESET state. In a second process step 1814, thesubsection is checked for initialization errors (that is, a cell whichhas not been placed into a RESET state and instead remains in a SETstate) and the process advances to step 1816. In step 1816, if therewere no initialization errors detected in step 1814, the processadvances to step 1850 without employing the DFEC pattern encodingmethods. Otherwise the process proceeds to step 1822, wherein the inputdata (that is, the data due to be written to the subsection) is comparedto the error cells. If all of the error cells detected in step 1814 aredue to be programmed into a SET state there is effectively no error, asthe cells are already in the desired final state. In this case theprocess advances to step 1850 without employing the DFEC patternencoding methods. If, however, at least one of the error cells(currently SET) are due to be programmed into a RESET state, then theDFEC methods may be used to correct at least one of the errors and theprocess proceeds to data pattern matching (DPM) step 1830. Here, theDFEC flag bits are activated according to the position of the error(s)in the block. Thus, if there is at least one even error, the even flagbit is activated, and if there is at least one odd error the odd flagbit is activated. If there are both even and odd errors present in thesubsection, then both flag bits will be activated. In some cases,however, wherein the number of effective errors is less than half of thenumber of total errors, the corresponding flag bit will remainun-activated. Thus, for example, if four total even errors occur butonly one is an effective error, the even flag bit may remainun-activated. In these situations, inverting the input data may increasethe number of errors.

In step 1840, the input data is encoded according to the patternindicated by the pattern reference code (PRC). If only the even flag isactivated then only the even input data bits are inverted, and if onlythe odd flag bit is activated then only the odd input data bits areinverted. If both even and odd flag bits are activated then all of theinput data will be inverted, similarly to the single-flag DFEC methoddiscussed previously. In this way, the input data is encoded to matchthe error pattern present in the array.

During process step 1850, the original input data (if there were noeffective errors), or the DFEC encoded input data, is further encodedusing a secondary error correction method (in this case, BCH ECC),including the DFEC flag cells. Next, the encoded input data is writtento the subsection in step 1860, as well as the BCH parity bits, whereincells requiring a logic “1” according to the encoded input data are SET.At this point, the array has been programmed with the input data and mayremain in this state until overwritten.

Next, in order to retrieve the input data, the process proceeds to step1870, wherein a READ operation is performed on the subsection and theREAD data is subsequently decoded using the secondary error correctionmethod (in this case, BCH ECC). In step 1870, any errors which occurredduring the SET operation of step 1860, or were not corrected by DFEC instep 1840, are corrected by the secondary error correction method.Following decoding, the DFEC flag bits are checked in step 1882. Ifneither flag bit is activated, then the process proceeds to step 1890and is complete. If at least one flag bit is active, however, theprocess advances to reverse data pattern matching (RDPM) step 1884. InRDPM step 1884, the output data is inverted according to the active DFECflags, or, the pattern reference code (PRC). Thus, if only the even flagbit is activated then only the even output bits are inverted, and ifonly the odd flag bit is activated then only the odd output data bitsare inverted. If both even and odd DFEC flag bits are activated, thenall of the output data bits are inverted. In this way, the originalinput data may be recovered and any errors corrected.

As discussed previously, the DRFEC method of the present disclosure maybe capable of correcting at least one odd and one even initializationerror per subsection. If the subsection contains multiple even ormultiple odd initialization errors, however, the dual-flag method maynot be able to correct all of the errors in the subsection. Further, theDFEC methods of the present disclosure are only capable of correctinginitialization errors. Therefore, a secondary error correction method,such as BCH, is required to correct programming errors, DFEC flagerrors, data retention errors, and any initialization errors that arenot corrected by the DFEC methods.

FIGS. 19A-19F illustrate the use of the dual-flag DFEC method of thepresent disclosure to correct various error patterns in an exemplaryresistive change element array subsection. It should be noted that thesubsection used in the examples of FIGS. 19A-19F is for illustrativepurposes only, and in real applications a subsection may contain a fargreater number of data bits than the four depicted. Further, as inprevious examples, the initialization state of the subsection is takento be RESET (logic “0”), but the invention is not limited in thisregard.

FIG. 19A depicts an exemplary programming/read operation 1901 in whichthe DRFEC method of the present disclosure is used to correct a singleodd error in a subsection. In a first operation 1910 a, analogous tooperations 1812, 1814, and 1816, all of the bits in the subsection(including both data and flag bits) are initialized to a RESET state.Data bit 1915 a, however, failed to initialize and remains in a SETstate. Since an error has been detected, the operation moves to step1920 a, wherein error cell 1915 a is compared to the input data 1925 a.As can be seen, data cell 1915 a needs to be in a RESET state. Thus, theoperation proceeds to data pattern matching (DPM) step 1930 a. In thisstep, the error cell is identified as an odd bit and the odd flag bit isactivated to indicate the presence of an odd error. Next, the input datais encoded according to the data pattern indicated by the activatedflags in step 1940 a. Since the odd flag bit is activated, all of theodd bits in the input data are inverted. The DFEC encoded input data andflag bits is then further encoded using a secondary error correctionmethod, such as BCH, in step 1950 a. The encoded input data is thenwritten to the subsection in step 1960 a. The subsection, aside fromerror cell 1915 a, is already in a RESET state, and thus only those bitswhich require a logic “1”, as per the encoded input data, need to beset. Error cell 1915 a is already in a SET state, however, and may notrequire programming, but in certain applications error cell 1915 a maybe set again to ensure it is fully in a SET state. At this stage, theencoded input data has been written to the subsection and theprogramming operation ends.

To retrieve the input data, a READ operation is performed, beginning atstep 1970 a. In step 1970 a, the memory block is read and the READ datasubsequently decoded using the secondary error correction method. Duringthis decoding process, any errors which occurred during the SEToperation of step 1960 a, as well as initialization errors not correctedby DFEC encoding, may be corrected by the secondary error correctionmethod. In this example, however, no such errors occurred, and theprocess proceeds to reverse data pattern matching (RDPM) step 1980 a.Here, the DFEC encoded output data is decoded according to the PRCindicated by the DFEC flags. In this case, the odd flag bit is activeand therefore all odd bits in the output data are inverted. At thispoint, since input data 1925 a has been recovered and odd initializationerror 1915 a has been corrected, programming/read operation 1901concludes. In this way, the dual-flag DFEC method of the presentdisclosure may be used to correct an odd initialization error.

Looking now to FIG. 19B, in an exemplary programming/read operation1902, the DRFEC method of the present disclosure is used to correct aneven error in a subsection. In a first operation 1910 b, analogous tooperations 1812, 1814, and 1816, all of the bits in the subsection(including both data and flag bits) are initialized to a RESET state.Even data bit 1916 b, however, failed to initialize and remains in a SETstate. Since an error has been detected, the operation moves to step1920 b, wherein error cell 1916 b is compared to the input data 1925 b.As can be seen, data cell 1916 b needs to be in a RESET state. Thus, theoperation proceeds to data pattern matching (DPM) step 1930 b. In thisstep, the error cell is identified as an even bit and the even flag bitis activated to indicate the presence of an even error. Next, the inputdata is encoded according to the PRC indicated by the activated flags instep 1940 b. Since the even flag bit is activated, all of the even bitsin the input data are inverted. The DFEC encoded input data and flagbits is then further encoded using a secondary error correction method,such as BCH, in step 1950 b. The encoded input data is then written tothe subsection in step 1960 b. The subsection, aside from error cell1916 b, is already in a RESET state, and thus only those bits whichrequire a logic “1”, as per the encoded input data, are set. Error cell1915 b is already in a SET state, however, and may not requireprogramming, but in certain applications error cell 1916 b may be setagain to ensure it is fully in a SET state. At this stage, the encodedinput data has been written to the subsection and the programmingoperation ends.

To retrieve the input data, a READ operation is performed, beginning atstep 1970 b. In step 1970 b, the memory block is read and the READ datasubsequently decoded using the secondary error correction method. Duringthis decoding process, any errors which occurred during the SEToperation of step 1960 b, as well as initialization errors not correctedby DFEC encoding, may be corrected by the secondary error correctionmethod. In this example, however, no such errors occurred, and theprocess proceeds to reverse data pattern matching (RDPM) step 1980 b.Here, the DFEC encoded output data is decoded according to the PRCindicated by the DFEC flags. In this case, the even flag bit is activeand therefore all even bits in the output data are inverted. At thispoint, since input data 1925 b has been recovered and eveninitialization error 1916 b has been corrected, programming/readoperation 1902 concludes. In this way, the dual-flag DFEC method of thepresent disclosure may be used to correct an even initialization error.

FIG. 19C illustrates the necessity of a secondary error correctionmethod to ensure that all errors in a subsection can be corrected duringa programming/read operation 1903. First, in step 1910 c, all cells inthe subsection are initialized to a RESET state. As can be seen, unlikethe examples of FIGS. 19A and 19B, no errors occurred in the subsectionduring the initialization process. Hence, input data 1925 c remainsunchanged, and the DFEC flag bits remain unactivated. Next, in step 1950c, the input data is encoded using a secondary error correction method,in this case BCH ECC, and then written to the subsection in step 1960 c.Since the subsection has been initialized to a RESET state, only thosecells which require a SET state need to be written. As can be seen,however, during set process 1960 c, cell 1917 c failed to set andremains in a RESET state. At this point, input data 1925 c has beenprogrammed to the subsection with one error, and will remain in thisstate until overwritten.

In order to retrieve input data 1925 c, a READ operation is performed onthe subsection. The read data is then decoded using the secondary errorcorrection method, in this case BCH, in step 1970 c. During the decodingprocess, error 1917 c is detected in the read data using the parity bitsand subsequently corrected by the secondary error correction method. Inthis way, input data 1925 c is recovered despite SET error 1917 c, whichcould not be corrected by the DFEC methods of the present disclosure, asthese methods may only correct errors occurring during initialization.

That said, however, the DFEC methods may be highly efficient atcorrecting initialization errors. FIG. 19D demonstrates the correctionof a continuous error (that is, two errors occurring on adjacent databits) using the dual-flag DFEC method of the present disclosure duringprogramming/read operation 1904. In a first step 1910 d, all the cellsin the subsection are initialized to a RESET state. Duringinitialization, however, a continuous error occurs, wherein two databits, 1915 d and 1916 d, fail to initialize and remain in a SET state.In step 1920 d, these errors are detected and subsequently compared toinput data 1925 d to check the desired final state of the error bits. Ascan be seen, error bits 1915 d and 1916 d need to be in a RESET stateand must be corrected.

Since initialization errors have been detected, the process proceeds toDPM step 1930 d. In this step, error 1915 d is determined to be an odderror, and error 1916 d is identified as an even error. Thus, both evenand odd DFEC flag bits are activated to indicate the presence of botheven and odd errors. Accordingly, in step 1940 d, both even and odd bitsin input data 1925 d are inverted according to the PRC indicated by theactivated flag bits. In this case, since both even and odd flag bits areactivated, all of the input data is inverted. Next, the DFEC encodedinput data, as well as the DFEC flag bits, are encoded using a secondaryerror correction method (in this case BCH ECC) in step 1950 d. Theencoded input data and DFEC flag bits are then written to the subsectionin step 1960 d. Since the subsection has been initialized to a RESETstate (aside from error cells 1915 d and 1916 d), only those cellsrequiring a SET state need to be written. Following the write process1960 d, the subsection has been programmed with the encoded input dataand may remain in this state until overwritten.

In order to retrieve input data 1925 d, a READ operation is performed onthe subsection. The read data is then decoded using the secondary errorcorrection method in step 1970 d. Since at least one DFEC flag bit isactivated, the read data is further decoded according to the PRCindicated by the active flag bits in reverse data pattern matching(RDPM) step 1980 d. Both even and odd flag bits are active, and thus allof the data bits in the read data are inverted to recover the originalinput data 1925 d, and the operation concludes. In this way, thedual-flag DFEC method of the present disclosure may be guaranteed tocorrect at least one odd and one even error per subsection using twoflag bits.

Looking now to FIG. 19E, the dual-flag DFEC method of the presentdisclosure is again employed to correct a continuous initializationerror. First, all of the bits in the subsection, including both data andflag bits, are initialized to a RESET state in step 1910 e. Two cells1915 e and 1916 e fail to initialize, however, and remain in a SETstate. In step 1920 e, error cells 1915 e and 1916 e are compared toinput data 1925 e to determine whether they are due to be in a SET orRESET state. As can be seen, error cell 1915 e needs to be in a RESETstate, while error cell 1916 e needs to be SET. Since error cell 1916 eis already in a SET state, there is effectively no error in this cell.Next, DPM step 1930 e activates the DFEC flag bits according to thedetected error pattern. Error cell 1915 e occurred on an odd bit whileeven error 1916 e does not need to be corrected, thus the odd flag bitis activated to indicate the presence of an odd error while the evenflag bit remains unactivated.

Input data 1925 e is then DFEC encoded according to the PRC indicated bythe activated flag bits. Only the odd flag bit is activated, and thusonly the odd bits in input data 1925 e are inverted. Once encodedaccording to the PRC, the DFEC encoded input data is further encodedusing a secondary error correction method (in this case BCH ECC) in step1950 e. The encoded input data is then written to the subsection inprogramming step 1960 e. Since the subsection has been initialized to aRESET state (aside from error bits 1915 e and 1916 e), only those cellsrequiring a SET state need to be written. Error bit 1916 e is already ina SET state, however, and may not need to be programmed, but in certainapplications error bit 1916 e may be programmed to ensure it is fully ina SET state. Following programming step 1960 e, the flag bits and inputdata 1925 e have been written to the subsection, and it may remain inthis state until overwritten.

In order to retrieve input data 1925 e, a READ operation is performed onthe subsection. The read data is then decoded using the secondary errorcorrection method (in this case, BCH ECC) in step 1970 e. Next, since atleast one DFEC flag bit is activated, the process proceeds to RDPM step1980 e, wherein the encoded read data is further decoded according tothe PRC indicated by the activated flags. Only the odd flag bit isactive, therefore all of the odd bits in the read data are inverted torecover the original input data 1925 e. At this point, input data 1925 ehas been recovered and the operation concludes. In this way, thedual-flag DFEC method of the present disclosure may correct at least oneinitialization error and determine whether an error must be corrected.

FIG. 19F demonstrates the use of the dual-flag DFEC method working inconcert with a secondary error correction method to correct multipleeven or odd errors in programming/read operation 1906. First, in step1910 f, the subsection is initialized to a RESET state. Data cells 1915f and 1918 f, however, fail to initialize and remain in a SET state.Next, in step 1920 f, error cells 1915 f and 1918 f are compared toinput data 1925 f to determine whether the errors need to be corrected.As can be seen, error cell 1915 f is due to be SET and does not need tobe corrected. Error cell 1918 f, however, needs to be in a RESET state.Since a correctable error has been detected, the process proceeds to DPMstep 1930 f, wherein the DFEC flag bits are activated according to theerror pattern detected. Correctable error 1918 f occurred on an odd databit, therefore only the odd flag bit is activated.

Input data 1925 f is then encoded according to the PRC indicated by theactivated flag bits in step 1940 f. Since the odd flag bit is activated,all of the odd bits in input data 1925 f are inverted. As can be seen,although this process corrects error 1918 f, error cell 1915 f is noweffectively an error, as it now needs to be in a RESET state rather thana SET state according to the DFEC encoded input data. As discussedpreviously, however, the DFEC methods of the present disclosure are onlycapable of correcting initialization errors, and thus cannot correcterror cell 1915 f.

Now that the input data has been DFEC encoded, input data 1925 f is thenfurther encoded using a secondary error correction method (in this case,BCH ECC) in step 1950 f. The encoded input data is then written to thesubsection in step 1960 f. The subsection has been initialized to aRESET state (aside from error cells 1915 f and 19180, however, and thusonly those cells requiring a SET state need to be written. As can beseen, although error cell 1918 f has been corrected, error cell 1915 fremains in a SET state despite needing to be in a RESET state, and isnow effectively an error. This demonstrates how the dual-flag DFECmethod may be unable to correct certain error patterns in a subsection,and, as will be described later, a more advanced DFEC method may berequired in applications where these patterns are expected to occurfrequently. At this point, input data 1925 f has been written to thesubsection with an error, and will remain in this state untiloverwritten.

In order to retrieve input data 1925 f, a READ operation is performed onthe subsection. The read data is first decoded using the secondary errorcorrection method (in this case, BCH ECC) in step 1970 f. During thisprocess, error 1915 f, which could not be corrected by the dual-flagDFEC method, is corrected by the secondary error correction method.Next, since at least one flag bit is activated, the read data is furtherdecoded in RDPM step 1980 f. Here, the read data is decoded according tothe PRC indicated by the activated flags. Only the odd flag bit isactivated, therefore all of the odd bits in the read data are invertedto recover the original input data 1925 f. In this way, the dual-flagDFEC method may be used in concert with a secondary error correctionmethod to correct multiple odd or even initialization errors in asubsection.

FIG. 20A is a simplified schematic diagram illustrating an exemplaryencoding circuit 2001 suitable for use with the dual-flag DFEC method ofthe present disclosure, which handles the error detection, data patternmatching (DPM) and encoding steps described above. As with otherexamples within the present disclosure, exemplary encoding circuit 2001is arranged for use with a resistive change element array that usesRESET (that is, logic “0”) as its initialization state. However, itshould be noted that, as previously discussed, the methods of thepresent disclosure are not limited in this regard. The DFEC methods ofthe present disclosure can also be used within a resistive changeelement array that uses a SET state as an initialization condition, andexemplary encoding circuit 2001 is intended only as a non-limiting,illustrative example of a circuit capable of encoding programming dataaccording to the DRFEC method of the present disclosure.

Looking now to FIG. 20A, an array of AND gates 2025 ANDs together thedata cells of a resistive change element array subsection 2010 withinput data 2015 (analogous to input data 1925 a-f in FIGS. 19A-19F)which is inverted through an array of Inverter gates 2020. Unlike theexemplary circuit of FIG. 12A, the outputs of AND gates 2025 are dividedbetween odd and even bits, depending on the position of the attachedcell in the subsection. The even bits are then ORed together with ORgate 2030 to provide even flag bit 2052 for the subsection 2010, whilethe odd bits are then ORed together with OR gate 2035 to provide oddflag bit 2055. Such a configuration will activate (that is, set to alogic “1” within this exemplary encoding circuit 2001) the appropriateflag bit(s) 2052 or 2055 if any data cell within subsection 2010 failsto initialize (that is, if any data cell is in a SET state) and isintended to be programmed into a RESET state, referred to as aneffective error. Thus, if at least one effective initialization erroroccurs on an even bit, even flag bit 2052 will be activated, while if atleast one effective initialization error occurs on an odd bit then oddflag bit 2055 will be activated. Such a logic function is analogous tooperations 1814, 1816, 1822, 1824, and 1830 within FIG. 18, discussed indetail with respect to that figure above.

Finally, an array of Exclusive OR gates 2040 selectively inverts theeven and odd input data 2015 dependent on the state of flag bits 2052and 2055. For each bit, the array of Exclusive OR gates 2040 thenprovides either the input data 2015 (if the corresponding flag bit 2052or 2055 is not activated) or an inverted version of the input data 2015(if the corresponding flag bit 2052 or 2055 is activated) to form theprogram data 2050, which will be programmed into the array in aprogramming operation. Such a logic function is analogous to operation1840 within FIG. 18, discussed in detail above.

FIG. 20B is a simplified schematic diagram illustrating an exemplarydecoding circuit 2002 suitable for use with the dual-flag DFEC method ofthe present disclosure, which handles the reverse data pattern matching(RDPM) and DFEC decoding steps described above. As with encoding circuit2001 within FIG. 20A as well as other examples within the presentdisclosure, exemplary decoding circuit 2002 is arranged for use with aresistive change element array that uses RESET (that is, logic “0”) asits initialization state. Again, it should be noted that, as previouslydiscussed, the methods of the present disclosure are not limited in thisregard. The DFEC methods of the present disclosure can also be usedwithin a resistive change element array that uses a SET state as aninitialization condition, and exemplary decoding circuit 2002 isintended only as a non-limiting, illustrative example of a circuitcapable of decoding program data according to the dual-flag DFEC methodof the present disclosure.

Looking now to FIG. 20B, an array of Exclusive OR gates 2080 selectivelyinverts read data 2060 (that is, data read out of an array subsection)dependent on the position of each bit and the state of the correspondingflag bit 2072 or 2075. Thus, each even bit in read data 2060 will beinverted if even flag bit 2072 is active, while each odd bit in readdata 2060 will be inverted if odd flag bit 2075 is active. The array ofExclusive OR gates 2080 then provides either the read data 2060 (if theflag bit 2072 or 2075 corresponding to the data bit is not activated) oran inverted version of the read data 2060 (if the flag bit 2072 or 2075corresponding to the data bit is activated) to form the output data 2090of the resistive change element array. Such a logic function isanalogous to operations 1882 and 1884 within FIG. 18, discussed indetail with respect to that figure above.

Advanced Bit Flip Error Correction

As described previously with respect to FIGS. 17A-17B, 18, and 19A-19F,the dual-flag DFEC method may be used to correct at least one odd andone even initialization error per subsection in a resistive changeelement array. This method, however, provides only a 75% chance ofcorrecting two initialization errors per subsection, due to thestatistical distribution of errors. Thus, in certain applicationswherein the error distribution is expected to result in multiple even orodd errors per subsection, the dual-flag DFEC method may beinsufficient. In these circumstances, a more advanced form of errorpattern matching using two or more flag bits per subsection may beemployed to increase the error correction rate.

To this end, the present disclosure teaches a third DFEC method usingtwo or more flag bits, referred to as advanced bit flip (ABF). Similarlyto the previously presented methods, ABF uses distributed flag bits todelimitate subsections within a resistive change element array.Following initialization, these distributed flags may be used to store apattern reference code (PRC) which identifies an encoding patternselected during a data pattern matching (DPM) step, wherein the patternof initialization errors within the subsection is matched to the inputdata pattern to select an encoding pattern capable of correcting themaximum number of initialization errors. The input data is then encodedso that at least some of the initialization errors may be corrected, ashas been described similarly with respect to the single-flag anddual-flag DFEC methods.

Unlike the RFEC or DRFEC methods, however, the ABF method divides thesubsections within a resistive change element array and the set of inputdata into equivalent data subsets comprising at least two data bits. Forexample, the 2-bit ABF method groups pairs of data bits in the array andthe input data into subsets of two bits, so that a set of input datahaving the pattern (00101110) would be divided into subsets of two bits(00|10|11|10). A 3-bit ABF method would divide a set of data intosubsets of three bits, while a 4-bit ABF method would divide a set ofdata into subsets of four bits. Thus, the same input pattern previouslydivided into four subsets (00|10|11|10) for the 2-bit method would thusbe divided into two subsets (0010|1110) for the 4-bit method.

As will be described in more detail later, encoding patterns may beidentified to translate one data pattern to a second data pattern. Asubset of input data having the data pattern 10, for instance, could betranslated to 00, 01, 10 or 11 before being written to the array. If a2-bit subset of data from an initialized array contains a single error,for instance, it may have a data pattern of either 10 or 01, and itscorresponding subset in the input data may have a data pattern of either00, 01, 10 or 11. It may be apparent that if the error pattern matchesthe input pattern there is effectively no error. Thus, an error patternof 10 having a corresponding input pattern of 10 or 11 may not beconsidered an effective error. From this, if an effective error doesoccur (error pattern 10 with input 00 or 01, for instance), the inputpattern may be translated according to a reversible encoding pattern tocorrect the error. For example, if an even error occurs (10) with aninput pattern of 00, 00 may be translated to 10 or 11 in order tocorrect the error. In this way, the ABF method of the present disclosuremay be used to correct initialization errors in a resistive changeelement array by employing a set of encoding patterns.

The ABF method therefore provides a much higher error correction rate byallowing more advanced data patterns to be identified. As describedpreviously, the number of flags employed for the DFEC methods determinesthe number of pattern codes which may be stored. Thus, a single flag bitprovides two patterns, two flag bits provides four patterns, three flagbits provides eight patterns, and n flag bits provide 2^(n) patterns.Further, encoding patterns may be identified for subsets comprising anynumber of data bits. For ease of explanation, however, the presentdisclosure describes the ABF method using subsets of two data bits, butit will be apparent to those skilled in the art that subsets comprisingany number of data bits may be employed.

It should be noted that, in the present disclosure, a “data subset” or“subset” may be used interchangeable to refer to two or more data bitswhich have been grouped together for the purposes of encoding anddecoding. As mentioned throughout, although described using two bits forease of explanation, a data subset may comprise any number of data bitsaccording to the requirements of a certain application. Further, a datasubset may be defined within the design of the resistive change elementarray itself, or may be defined by a processing element, encodingcircuit, decoding circuit, or whatever means best befits a particularapplication. That is, so long as a means to divide a set of input dataand a set of read data into subsets suitable for encoding and decodingis provided, the invention is not limited in the specific methodemployed nor in the number of bits grouped into a data subset.

Looking now to FIG. 21A, eight exemplary reversible data patterns 2110used in the 2-bit ABF method are displayed. Each pattern 2110 has anassociated pattern reference code (PRC) 2120 which, when stored in theDFEC flags, indicate which pattern must be employed for DFEC encoding ofthe input data and decoding of the read data. As can be seen, theencoding process is achieved by selectively inverting bits in a datasubset, while the decoding process simply reverses the pattern.Specifically, the “before” columns correspond to the possible patternsfor each data subset in the input data, while the “after” columnsrepresent the corresponding encoded pattern for each input data subset.For example, according to pattern 4, a pair of bits 00 in the input datawill be encoded as 11 when written to the subsection, while a pair ofbits 01 in the input data will be encoded as 10. In this way, the inputdata may be encoded and decoded according to an encoding pattern inorder to match the pattern of errors to the input data.

In this way, the ABF method utilizes reversible translation patterns toencode input data for the purposes of error correction. Specifically,these encoding patterns provide a reversible means to translate everypossible logical configuration of bits in a data subset to a separatelogical configuration without overlap. Mathematically, the number ofpossible configurations for a given number of bits in a data subset canbe calculated as 2^(n), where n is the number of bits. Thus, a singlebit can have two configurations (1 or 0), two bits can have fourcombinations (00, 10, 01 and 11), three bits can have eightcombinations, and four bits can have sixteen configurations. Since eachconfiguration must be matched with only a single encoded configurationwithout overlap to ensure reversibility (for example, an input 11 cannotbe translated as both 01 and 10, and input patterns 00 and 10 cannotboth translate to 01), the total number of possible encoding patternsmay be calculated as 2^(n)!, where n is the number of bits in a subset.Thus, for two bits there are 24 (2²!=4!=4*3*2*1=24) total encodingpatterns, for three bits there are 40,320 (8!) total patterns, and forfour bits there are nearly 21 trillion (16!) total patterns.

As will be discussed in more depth later, the number of bits chosen toform a subset may substantially affect the error correction rate. Thenumber of possible encoding patterns increases quickly, however, andeach pattern must be provided with a pattern reference code (PRC). Sincea given number of flag bits n can store 2^(n) pattern reference codes,in order to utilize all possible encoding patterns for the 2-bit method,for instance, requires 5 flags (log₂(24)=5) to reference all 24patterns. The 3-bit ABF method, which has 40,320 encoding patterns,requires 16 flags (log₂(40,320)), and the 4-bit ABF method requires 45.Increasing the number of bits in a subset may therefore increase theflag bit overhead, but may also improve the error correction rate byallowing a greater percentage of errors to be corrected. Further, whilethe total set of encoding patterns guarantees the ability to translateeach input pattern to any output pattern, not all of these patterns maybe necessary for certain applications. Therefore, in certainapplications it may be advantageous to select a smaller set of the totalset of encoding patterns (such as the eight patterns depicted in FIG.21A) to simplify the encoding and decoding process.

In these cases, a smaller set of encoding patterns may be selected fromthe total mathematically possible set of encoding patterns according tocertain criteria. To guarantee the correction of at least twoinitialization errors per subsection using 2-bit data subsets, forinstance, requires at least eight encoding patterns selected accordingto the following conditions from the set of 24 patterns shown in FIG.21B. First, it must be possible to translate all possible input datapatterns (00, 10, 01, and 11) to data pattern ‘XX’, where X is theselected initialization state, in order to correct a data subsetcontaining a continuous error (a data subset with two initializationerrors). Second, any combination of two input data patterns within allpossible data patterns (00, 10, 01, and 11) must be capable of beingencoded to all four possible combinations of two errors in two datasubsets (10 and 10, 01 and 01, 10 and 01, and 01 and 10). A set ofencoding patterns meeting these requirements may be guaranteed tocorrect up to two errors per subsection, such as the set of 8 patternsdepicted in FIG. 21A. If more than two errors occur, however, thesepatterns may still be able to correct all the errors (as shown in FIG.24F below) dependent on the error and input data patterns. It will beapparent that using a larger number of flag bits allows a greater numberof encoding patterns to be selected. These additional patterns mayprovide a greater ability to address more than two errors, should theyoccur, as will be discussed in more detail below. Additionally, thecriteria by which the encoding patterns are selected may vary. Forinstance, a set of six encoding patterns could be defined to guaranteeall input data subsets with a pattern 00 are translated to 11, or allinput subsets with 01 are translated to 10. The selection of the eightpatterns shown in FIG. 21A is therefore provided only as an example, andsuch a set of encoding patterns can be defined by whatever criteria mayprovide a desired encoding result.

Further, although demonstrated using 2-bit ABF for ease of explanation,the number of bits grouped together for the purposes of data patternmatching and DFEC encoding may be selected according to the needs of aspecific application. Indeed, it will be apparent to those skilled inthe art that encoding patterns may be identified for groups of threedata bits, or even larger groups of four or more bits. The subset size,set of encoding patterns, and number of flags employed for the ABFmethod must be optimized for a given application and are not limited tothe examples herein.

FIG. 22A depicts an exemplary resistive change element array 2201configured for use with the three-flag ABF method of the presentdisclosure in concert with a secondary error correction method, in thiscase BCH ECC. Resistive change element array 2201 comprises multiple, inthis example, eight, BCH ECC memory blocks 2210, which are illustratedin more detail in FIG. 22B. Each memory block 2210 is divided intoseveral, in this example, four, DFEC subsections 2211, which comprisedata bits 2211 a and three DFEC flag bits 2211 b. Each memory block 2210additionally comprises BCH parity bits 2212. Unlike the previousexamples discussed in FIGS. 9A-9B and 17A-17B, data bits 2211 b may bedivided into data subsets in order to employ the patterns depicted inFIG. 21 above. These data subsets may be defined by hardware or may bedefined during the encoding process, the invention is not limited inthis regard. Further, it will be apparent to those skilled in the artthat the number of data bits and data subsets per subsection isarbitrary and may be substantially large in real applications.Initialization errors may occur throughout resistive change elementarray 2201, including data bit errors 2220 which occur in datasubsections 2211 a, BCH parity errors 2230 which occur in the BCH paritybits 2212, and DFEC flag errors 2240 which occur in DFEC flag bits 2211b. As described previously, the three DFEC flag bits 2211 b insubsections 2211 are capable of correcting at least two data bitinitialization errors 2220 per subsection 2211. DFEC flag bitinitialization errors 2240, BCH parity bit errors 2230, and all othererrors must be corrected by a secondary error correction method. The useof the ABF method, however, may substantially reduce the number oferrors required to be corrected by this secondary error correctionmethod for a given BER. Since the secondary error correction methodsrequire many parity bits 2212 to correct a single error (BCH ECCrequires 14 parity bits/error, for instance), compared to three-flag2-bit ABF method of the present disclosure which requires three flagbits to correct two or more data bit initialization errors 2220, the useof the ABF methods of the present disclosure may substantially reducethe parity overhead for a given correctable BER.

FIG. 22B depicts the exemplary memory block 2210 of FIG. 22A in moredetail. As can be seen, memory block 2210 comprises multiple, in thiscase, four, subsections 2211, as well as parity bits 2212 for use with aselected secondary error correction method (such as, but not limited to,BCH). Subsections 2211 comprise data bits 2211 a and DFEC flag bits 2211b. In this example, memory block 2210 is configured for the three-flagABF method and hence DFEC flag bits 2211 b comprise three flag bits persubsection. As described previously, however, a greater number of flagbits may be employed in certain applications in order to store a largernumber of pattern reference codes. Further, to facilitate ABF patternencoding, data bits 2211 a are divided into data subsets, which maycomprise two or more data bits.

FIG. 23 is a flowchart depicting the use of the advanced bit flip (ABF)methods of the present disclosure during a programming/read operation2300 on a single subsection of a resistive change element array, such asthat depicted in FIG. 22A. In this example, the initialization state ofthe array is taken to be RESET (or, logic “0”), but as describedpreviously the invention is not limited in this regard. In a firstprocess step 2312 the subsection is initialized, wherein all cells(including both DFEC flags and data cells) are placed into a RESETstate. In a second process step 2314, the subsection is checked forinitialization errors (that is, a cell which has not been placed into aRESET state and instead remains in a SET state) and the process advancesto step 2316. In step 2316, if there were no initialization errorsdetected in step 2314, the process advances to step 2350 withoutemploying the DFEC pattern encoding methods. Otherwise the processproceeds to step 2322, wherein the input data (that is, the data due tobe written to the subsection) is compared to the error cells. If all ofthe error cells detected in step 2314 are due to be programmed into aSET state there is effectively no error, as the cells are already in thedesired final state. In this case the process advances to step 2350without employing the DFEC pattern encoding methods. If, however, atleast one of the error cells (currently SET) are due to be programmedinto a RESET state, then the DFEC methods may be used to correct atleast one of the errors, and the process proceeds to data patternmatching (DPM) step 2330. Here, the error pattern present in the arrayis compared to the input data pattern. An appropriate encoding pattern,such as those depicted in FIG. 21, is selected to match the input datato the initialization errors in order to correct the errors. The flagcells are then activated according to the pattern reference code (PRC)corresponding to the matched data pattern in step 2340.

Next, in step 2345, the input data is encoded according to the patternindicated by the pattern reference code (PRC) indicated by the DFEC flagcells. For example, if the flag cells are activated in the 101 pattern,corresponding to pattern 6 (as shown in FIG. 21), then all data subsetsin the input data with the pattern 10 are encoded as 01, and all datasubsets in the input data with the pattern 10 are encoded as 01, whiledata subsets with the pattern 00 or 11 remain unchanged, as will beillustrated in FIGS. 24A-24F. In this way, the input data is encoded tomatch the error pattern present in the array.

During process step 2350, the original input data (if there were noeffective errors), or the DFEC encoded input data, is further encodedusing a secondary error correction method (in this case, BCH ECC),including the DFEC flag cells. Next, the encoded input data, as well asthe BCH parity bits, are written to the subsection in step 2360, whereincells requiring a logic “1” according to the DFEC encoded input data areSET. At this point, the array has been programmed with the input dataand may remain in this state until overwritten.

Next, in order to retrieve the input data, the process proceeds to step2370, wherein a READ operation is performed on the subsection and theREAD data is subsequently decoded using the secondary error correctionmethod (in this case, BCH ECC). In step 2370, any errors which occurredduring the SET operation of step 2360, errors which were not correctedby DFEC in step 2345, as well as data retention errors, are corrected bythe secondary error correction method. Following BCH decoding, the DFECflag bits are checked in step 2382. If no flag bits are activated(corresponding to pattern 1), then the process proceeds to step 2390 andis complete. If at least one flag bit is active, however, the processadvances to reverse data pattern matching (RDPM) step 2384. In RDPM step2384, the output data is decoded according to the pattern reference code(PRC) indicated by the active DFEC flags. Thus, if the DFEC flagsindicate pattern 6 encoding, then all data subsets in the read data withthe pattern 01 are decoded as 10, and all data subsets in the read datawith the pattern 10 are decoded as 01, while all data subsets with thepattern 00 or 11 remain unchanged, as will be illustrated in FIGS.24A-24F. In this way, the original input data may be recovered and anyerrors corrected.

Three-Flag 2-Bit Advanced Bit Flip

As discussed previously, a set of eight encoding patterns (such as theeight patterns depicted in FIG. 21A) for the ABF methods of the presentdisclosure may be selected to guarantee the correction of at least twoinitialization errors per subsection. If the subsection contains morethan two initialization errors, however, this set of ABF encodingpatterns may not be able to correct all of the errors in the subsection.Further, all of the DFEC methods of the present disclosure, includingthe ABF methods, are only capable of correcting initialization errors.Therefore, a secondary error correction method, such as BCH, is requiredto correct programming errors, DFEC flag errors, and any initializationerrors that are not corrected by the DFEC methods.

The following examples of FIGS. 24A-24F discuss the use of thethree-flag 2-bit ABF method of the present disclosure. That is, theseexamples divide a subsection and its corresponding input data intosubsets of two bits while employing three flag bits per subsection tostore a pattern reference code (PRC) to indicate one of eight encodingpatterns. As discussed previously, this set of eight patterns may bepre-selected from the total set of encoding patterns for a given subsetsize (see FIG. 21B for the possible encoding patterns for the 2-bitmethod) according to the needs of a particular application. For ease ofexplanation, the examples of FIGS. 24A-24F employ the eight patternsshown in FIG. 21A, which are selected to ensure the correction of atleast two initialization errors per subsection and assume aninitialization state of logic ‘1’. The set of eight patterns shown inFIG. 21A, however, are provided only as a demonstration of the abilityto select a subset of encoding patterns according to a particular set ofrequirements, and the invention is not limited to the set of eightpatterns shown in FIG. 21A or in the selection criteria employed tochoose the set of encoding patterns. Further examples for a five-flag2-bit ABF method utilizing the total set of encoding patterns (see FIG.21B) may be found below in FIGS. 25A-25D.

It should be noted that the subsection used in the examples of FIGS.24A-24F is for illustrative purposes only, and in real applications sucha subsection may contain a far greater number of data bits than thesixteen depicted. Further, although the data cells have been assignedinto subsets of two bits for ease of explanation, the data cells may notnecessarily be grouped in the hardware of the array layout, and may begrouped by whatever means or in whatever number or pattern befits aparticular application. As in previous examples, the initializationstate of the subsection is taken to be RESET (logic “0”), but theinvention is not limited in this regard.

FIG. 24A depicts an exemplary programming/read operation 2401 using thethree-flag 2-bit ABF method of the present disclosure to correct twoeven initialization errors in subsection 2411 a. In a first process step2410 a, all of the cells in subsection 2411 a, including flag bits 2412a and data bits 2413 a, are initialized into a RESET state, analogous tooperation 2312. Next, the subsection is checked for initializationerrors in step 2415 a. As can be seen, two even cells 2416 a have failedto RESET and remain in a SET state, and the process proceeds to step2420 a. Here, analogous to data pattern matching (DPM) step 2330, theerror pattern is compared to input data 2422 a. As can be seen, thefirst error pair 10 already corresponds to the input data subset 10 andshould not be changed. The second error pair 10, however, needs tocorrespond to 00 in the input data. Thus, pattern 3 is selected to matchthe input data to the error pattern so as to translate 00 to 11 (thuscorrecting error 10) and translating 10 as 10 (correcting error 10).

Once the appropriate pattern has been identified, the DFEC flags areactivated in step 2425 a (analogous to operation 2340) according to thepattern reference code (PRC), which is 010 for pattern 3. Next, in step2430 a (analogous to operation 2345), the input data is encoded with theselected pattern, wherein each input data subset is selectively invertedaccording to the encoding pattern. The DFEC encoded input data and flagcells are then further encoded using BCH ECC in step 2440 a.

The encoded input data is then written to the subsection in step 2450 a.Since the subsection has already been initialized into a RESET state(aside from error cells 2416 a), only those cells requiring a logic 1according to the encoded input data need to be SET. In someapplications, error cells 2416 a do not need to be programmed, as theyare already in the correct state, however, in other applications theerror cells may be programmed to ensure they are fully in a SET state.Following this operation, the encoded input data has been written to thesubsection and RESET errors 2416 a have been corrected.

In order to retrieve input data 2422 a, a READ operation is performed onthe subsection. The read data is then decoded using BCH ECC in step 2460a. Since the read data has been encoded using the ABF method, the readdata must be further decoded in reverse data pattern matching (RDPM)step 2470 a. Here, the PRC indicated by the activated flag bits is usedto identify which ABF pattern is required to decode the read data. Thedata subsets within the read data are then selectively invertedaccording to the ABF pattern, which in this case is pattern 3. Oncedecoded, the read data matches the input data and the operationconcludes in step 2480 a. In this way, the three-flag 2-bit ABF methodof the present disclosure may be used to correct two even initializationerrors in a subsection.

Similarly, FIG. 24B depicts an exemplary programming/read operation 2402which demonstrates the use of the three-flag 2-bit ABF method of thepresent disclosure to correct two odd initialization errors in asubsection. In a first process step 2410 a, all of the cells insubsection 2411 b, including flag bits 2412 b and data bits 2413 b, areinitialized into a RESET state, analogous to operation 2312. Next, thesubsection is checked for initialization errors in step 2415 b. As canbe seen, two odd cells 2416 b have failed to RESET and remain in a SETstate, and the process proceeds to step 2420 b. Here, analogous to datapattern matching (DPM) step 2330, the error pattern is compared to inputdata 2422 b. As can be seen, both error pairs have the pattern 01, whileboth corresponding input data pairs require the pattern 10. Thus,pattern 5 is selected to match the input data 2422 b to the errorpattern by translating 10 to 01.

Once the appropriate pattern has been identified, the DFEC flags areactivated in step 2425 b (analogous to operation 2340) according to thepattern reference code (PRC), which is 011 for pattern 5. Next, in step2430 b (analogous to operation 2345), the input data is encoded with theselected pattern, wherein each input data subset is selectively invertedaccording to the encoding pattern. The DFEC encoded input data and flagcells are then further encoded using BCH ECC in step 2440 b.

The encoded input data is then written to the subsection in step 2450 b.Since the subsection has already been initialized into a RESET state(aside from error cells 2416 b), only those cells requiring a logic 1according to the encoded input data need to be SET. In someapplications, error cells 2416 b do not need to be programmed, as theyare already in the correct state, however, in other applications theerror cells may be programmed to ensure they are fully in a SET state.Following this operation, the encoded input data has been written to thesubsection and RESET errors 2416 b have been corrected.

In order to retrieve input data 2422 b, a READ operation is performed onthe subsection. The read data is then decoded using BCH ECC in step 2460b. Since the read data has been encoded using the ABF method, the readdata must be further decoded in reverse data pattern matching (RDPM)step 2470 b. Here, the PRC indicated by the activated flag bits is usedto identify which ABF pattern is required to decode the read data. Thedata subsets within the read data are then selectively invertedaccording to the ABF pattern, which in this case is pattern 5. Oncedecoded, the read data matches the input data and the operationconcludes in step 2480 b. In this way, the three-flag 2-bit ABF methodof the present disclosure may be used to correct two odd initializationerrors in a subsection.

Looking now to FIG. 24C, as discussed previously, the DFEC methods ofthe present disclosure are only capable of correcting initializationerrors. Here, the necessity of a secondary error correction method isdemonstrated through exemplary programming/read operation 2403. In afirst process step 2410 c, all of the cells in subsection 2411 c,including flag bits 2412 c and data bits 2413 c, are initialized into aRESET state, analogous to operation 2312. Next, the subsection ischecked for initialization errors in step 2415 c. As can be seen, noinitialization errors have occurred. In some applications, the processcontinues to step 2440 c without employing the DFEC methods.Alternatively, data pattern matching may be performed in step 2420 c tocompare the error pattern to input data 2422 c. In this case, sincethere are no effective errors, pattern 1 is selected and the flag bits2412 c are activated according to the PRC, which is 000.

In either case, the unchanged input data 2422 c is encoded using BCH ECCin step 2440 c. The encoded input data is then written to the subsectionin step 2450 c. Since the subsection has already been initialized into aRESET state, only those cells requiring a logic 1 according to theencoded input data need to be SET. During the writing process, however,SET error 2442 c occurs, and cell 2442 c remains in a RESET state.Following this operation, the encoded input data has been written to thesubsection with error 2442 c and may remain in this state untiloverwritten.

In order to retrieve input data 2422 c, a READ operation is performed onsubsection 2411 c. The read data is then decoded using BCH ECC in step2460 c. During this decoding process, the read data is compared to theBCH ECC parity bits to identify and correct error 2442 c. At this point,since there were no initialization errors, and programming error 2442 chas been corrected, the original input data 2422 c is recovered and theoperation may conclude in step 2480 c with no further actions taken. Insome applications, however, the BCH ECC decoded read data may be furtherdecoded using the ABF method in RDPM step 2470 c according to the PRCindicated by the activated flags. Since PRC encoded in the flags is 000,pattern 1 is identified and no changes need to be made to the BCH ECCdecoded read data. The input data is now recovered, and the operationconcludes in step 2480 c. In this way, the three-flag 2-bit ABF methodof the present disclosure may be used in concert with a secondary errorcorrection method to correct all types of errors.

FIG. 24D provides another example of the use of the three-flag 2-bit ABFmethod of the present disclosure to correct multiple errors in asubsection. In a first process step 2410 d, all of the cells insubsection 2411 d, including flag bits 2412 d and data bits 2413 d, areinitialized into a RESET state, analogous to operation 2312. Next, thesubsection is checked for initialization errors in step 2415 d. As canbe seen, two initialization errors 2416 d have occurred on one even andone odd cell, which remain in a SET state. In DPM step 2420 d, analogousto data pattern matching (DPM) step 2330, the error pattern is comparedto input data 2422 d. As can be seen, the first error pair has thepattern 01 and needs to be in the final state 11, so there iseffectively no error. The second error pair, however, has the pattern 10and needs to be in the final state 01. Thus, pattern 7 is selected tomatch the input data 2422 d to the error pattern by translating 01 to 11and 11 to 01.

Once the appropriate pattern has been identified, the DFEC flags areactivated in step 2425 d (analogous to operation 2340) according to thepattern reference code (PRC), which is 110 for pattern 7. Next, in step2430 d (analogous to operation 2345), the input data is encoded with theselected pattern, wherein each input data subset is selectively invertedaccording to the encoding pattern. The DFEC encoded input data and flagcells are then further encoded using BCH ECC in step 2440 d.

The encoded input data is then written to the subsection in step 2450 d.Since the subsection has already been initialized into a RESET state(aside from error cells 2416 d), only those cells requiring a logic 1according to the encoded input data need to be SET. In someapplications, error cells 2416 d do not need to be programmed, as theyare already in the correct state, however, in other applications theerror cells may be programmed to ensure they are fully in a SET state.Following this operation, the encoded input data has been written to thesubsection and RESET errors 2416 d have been corrected.

In order to retrieve input data 2422 d, a READ operation is performed onthe subsection. The read data is then decoded using BCH ECC in step 2460d. Since the read data has been encoded using the ABF method, the readdata must be further decoded in reverse data pattern matching (RDPM)step 2470 d. Here, the PRC indicated by the activated flag bits is usedto identify which ABF pattern is required to decode the read data. Thedata subsets within the read data are then selectively invertedaccording to the ABF pattern, which in this case is pattern 7. Oncedecoded, the read data matches the input data and the operationconcludes in step 2480 d. In this way, the three-flag 2-bit ABF methodof the present disclosure may be used to correct one odd and one eveninitialization error in a subsection.

Similarly, FIG. 24E depicts an exemplary programming/read operation 2405which again demonstrates the use of the three-flag 2-bit ABF method ofthe present disclosure to correct one odd and one even error in asubsection. In a first process step 2410 e, all of the cells insubsection 2411 e, including flag bits 2412 e and data bits 2413 e, areinitialized into a RESET state, analogous to operation 2312. Next, thesubsection is checked for initialization errors in step 2415 e. As canbe seen, one odd and one even cell 2416 e have failed to RESET andremain in a SET state, and the process proceeds to step 2420 e. Here,analogous to data pattern matching (DPM) step 2330, the error pattern iscompared to input data 2422 e. As can be seen, the first error pair hasthe pattern 10 and must be corrected to 01. The second error pair,however, has the pattern 01 and must be corrected to 10. Thus, pattern 2is selected to match the input data 2422 e to the error pattern bytranslating 01 to 10 and 10 to 01.

Once the appropriate pattern has been identified, the DFEC flags areactivated in step 2425 e (analogous to operation 2340) according to thepattern reference code (PRC), which is 001 for pattern 2. Next, in step2430 e (analogous to operation 2345), the input data is encoded with theselected pattern, wherein each input data subset is selectively invertedaccording to the encoding pattern. The DFEC encoded input data and flagcells are then further encoded using BCH ECC in step 2440 e.

The encoded input data is then written to the subsection in step 2450 e.Since the subsection has already been initialized into a RESET state(aside from error cells 2416 e), only those cells requiring a logic 1according to the encoded input data need to be SET. In someapplications, error cells 2416 e do not need to be programmed, as theyare already in the correct state, however, in other applications theerror cells may be programmed to ensure they are fully in a SET state.Following this operation, the encoded input data has been written to thesubsection and RESET errors 2416 e have been corrected.

In order to retrieve input data 2422 e, a READ operation is performed onthe subsection. The read data is then decoded using BCH ECC in step 2460e. Since the read data has been encoded using the ABF method, the readdata must be further decoded in reverse data pattern matching (RDPM)step 2470 e. Here, the PRC indicated by the activated flag bits is usedto identify which ABF pattern is required to decode the read data. Thedata subsets within the read data are then selectively invertedaccording to the ABF pattern, which in this case is pattern 2. Oncedecoded, the read data matches the input data and the operationconcludes in step 2480 e. In this way, the three-flag 2-bit ABF methodof the present disclosure may be used to correct one even and one oddinitialization error in a subsection.

In a final example, FIG. 24F depicts an exemplary programming/readoperation 2406 which demonstrates the ability of the three-flag 2-bitABF method of the present disclosure to correct more than two errors ina subsection, dependent on the error distribution. In a first processstep 2410 f, all of the cells in subsection 2411 f, including flag bits2412 f and data bits 2413 f, are initialized into a RESET state,analogous to operation 2312. Next, the subsection is checked forinitialization errors in step 2415 f. As can be seen, three cells 2416 fhave failed to RESET and remain in a SET state, and the process proceedsto step 2420 f. Here, analogous to data pattern matching (DPM) step2330, the error pattern is compared to input data 2422 f. As can beseen, the first error pair has the pattern 11 and must be corrected to10. The second error pair, however, has the pattern 01 and there iseffectively no error. Although the exemplary set of eight patterns shownin FIG. 21 are only guaranteed to correct up to two errors persubsection, in this case, pattern 8 is capable of correcting all threeerrors that have occurred due to the distribution of errors and theinput data pattern. Thus, pattern 8 is selected to encode the inputdata.

Now that the appropriate pattern has been identified, the DFEC flags areactivated in step 2425 f (analogous to operation 2340) according to thepattern reference code (PRC), which is 111 for pattern 8. Next, in step2430 f (analogous to operation 2345), the input data is encoded with theselected pattern, wherein each input data subset is selectively invertedaccording to the encoding pattern. The DFEC encoded input data and flagcells are then further encoded using BCH ECC in step 2440 f.

The encoded input data is then written to the subsection in step 2450 f.Since the subsection has already been initialized into a RESET state(aside from error cells 2416 f), only those cells requiring a logic 1according to the encoded input data need to be SET. In someapplications, error cells 2416 f do not need to be programmed, as theyare already in the correct state, however, in other applications theerror cells may be programmed to ensure they are fully in a SET state.Unfortunately, since programming operation 2450 f only SETs the cells,one of the errors 2416 f remains uncorrected by the ABF method, as itneeds to be RESET and is currently in a SET state. Following thisoperation, the encoded input data has been written to the subsectionwith an uncorrected initialization error 2452 f and may remain in thisstate until overwritten.

In order to retrieve input data 2422 f, a READ operation is performed onthe subsection. The read data is then decoded using BCH ECC in step 2460f. During the BCH ECC decoding process, the uncorrected error 2452 f isdetected and corrected by BCH ECC. Since the read data has also beenencoded using the ABF method, the read data must be further decoded inreverse data pattern matching (RDPM) step 2470 f. Here, the PRCindicated by the activated flag bits is used to identify which ABFpattern is required to decode the read data. The data subsets within theread data are then selectively inverted according to the ABF pattern,which in this case is pattern 8. Once decoded, the read data matches theinput data and the operation concludes in step 2480 f. In this way, thethree-flag 2-bit ABF method of the present disclosure may be used tocorrect multiple initialization errors in a subsection in concert with asecondary error correction method.

Five-Flag 2-Bit Advanced Bit Flip

As discussed previously, in some circumstances it may be advantageous toutilize all of the possible encoding patterns for a given subset size,despite the larger number of flag bits required. While the set of eightencoding patterns depicted in FIG. 21A are capable of correcting atleast two initialization errors per subsection, any errors beyond thesetwo are not considered during data pattern matching and the eightpatterns may not be sufficient to address all error patterns. Thus, thechances of being able to correct more than two errors per subsection islower than if all possible patterns are employed.

Statistically, assuming a large number of trials and a large number oferrors, using all 24 encoding patterns of the 2-bit ABF method canensure the correction of at least 50% of all errors in a subsection inaddition to the two errors which are guaranteed to be corrected (sincethe 24 patterns of FIG. 21B include the 8 patterns of FIG. 21A).Essentially, the probability of a continuous error (that is, twoinitialization errors in a single subset) is extremely small (roughly0.3%) and all errors may therefore be assumed to be either even or odd(that is, to have a data pattern of 10 or 01, respectively) and will bedistributed across the possible input patterns (00, 01, 10 or 11).Further, for the purposes of data pattern matching, a continuous errormay be treated as both an even and an odd error without being afforded aseparate category. From this, it can be seen that any input patterntranslated to 11 will be capable of correcting 100% of its correspondingerrors (three errors with pattern 10, 01, and 10 with correspondinginput pattern 00 can all be corrected if 00 is translated to 11, forinstance). Any input pattern translated to 10 or 01, on the other hand,only has a 50% chance of correcting its corresponding errors in theworst case distribution of errors. Only half set of a set of errors 01,01, 10 and 10 with corresponding input 11 can be corrected if 11 istranslated to 01 or 10, for instance. Usually, however, errors will notbe evenly distributed, and any imbalance can improve the number oferrors corrected above the minimum of 50%. Finally, any input patterntranslated to 00 has a 0% chance to correct any errors.

From this, assuming a worst-case distribution of errors (that is, anequal distribution of errors across input patterns), the minimum numberof errors which can be corrected may be calculated by:

${\# \mspace{14mu} {errors}\mspace{14mu} {corrected}} = {{{100\% \frac{n}{4}} + {50\% \frac{n}{4}} + {50\% \frac{n}{4}}} = {0.5\; n}}$

where n is the total number of initialization errors, giving aworst-case error correction of 50%. Unlike the three-flag method,however, in which three flags are used to correct at least two errors,the five flag ABF method is generally more efficient with largersubsections and higher error rates since its correction ability scalesas a percentage, but is not necessarily more effective. For example, if30 initialization errors occur in an 8 kb subsection, at least 15 errorsmay be corrected by five flag bits, giving three errors corrected perflag bit. This may be compared to the 14 parity bits required by BCH tocorrect a single error in a similar subsection. To illustrate this,FIGS. 25A-25C show calculations of the parity overhead for the five flagmethod at different BER and subsection size. It should be noted that theexamples of FIGS. 25A-25C are provided as an example of the advantagesand disadvantages and general trends of the various DFEC methods of thepresent disclosure and do not reflect experimental data or expectedoutcomes. The optimization of the DFEC methods taught herein must beperformed taking the particular conditions and requirements of aspecific application into account.

FIG. 25A shows a plot 2500 of the variation of parity overhead withsubsection size at five error rates in an exemplary 8 kb memory blockusing the five-flag 2-bit ABF method. As seen in previous examples ofthe DFEC methods, the parity overhead reaches a minimum at a particularnumber of subsections for each BER, increasing rapidly as the block isdivided into more subsections. This behavior is similar to that seen inFIGS. 16B and 16C, wherein the parity overhead reaches a minimum andthen increases as more subsections are added. This demonstrates the needto optimize the subsection configuration for the expected BER of aparticular application. In the case of the five-flag ABF method,however, the optimal number of subsections at each BER is greatlyreduced compared to the previous methods as a result of its particularscaling behavior, as described previously.

FIG. 25B provides a comparison between the parity overhead required forthe RFEC, DRFEC, three-flag 2-bit ABF and five-flag 2-bit ABF methods toachieve a correctable BER of 0.2% for a given number of subsections inan exemplary 8 kb memory block. At eight subsections, for instance, thefive-flag ABF method provides the lowest overall parity overhead, at1.05%. This may be compared to the parity overhead of 2.73% required forBCH alone, giving a reduction of nearly 62%. At smaller subsections,however, the five-flag method quickly loses efficiency, leading to aparity overhead of 4.47% at 64 subsections—larger than that required forBCH alone. It is thus imperative to select the DFEC method and theparticular configuration thereof to meet the requirements of a givenapplication.

In order to examine the relative strengths of each DFEC method,therefore, FIG. 25C provides a comparison of the optimal configurationof each DFEC method for five correctable error rates. First, the optimalsubsection configuration was calculated for each method. It may clearlybe seen that the five-flag ABF method requires a significantly reducednumber of subsections compared to the other DFEC methods, requiring onlya 16 subsections at 0.5% BER compared to the 32 for three-flag ABF, forinstance. Again, as discussed previously, this behavior is a result ofthe error correction scaling of the five-flag ABF method, which providesa guaranteed percentage rather than a discrete number of errorscorrected.

Looking to the next five columns, the parity overhead corresponding toeach of the optimal configurations was calculated for each error rate.As can be seen, the five-flag ABF method provides the most efficientsolution for all BERs. That said, however, at higher BERs of 0.4% and0.5%, the parity overhead required for the three-flag ABF method isnearly equal to that of the five-flag method. Since the three-flagmethod may employ a simpler circuit and reduced processing latency, inthese cases it may be advantageous to use only three-flags with aproperly selected subset of encoding patterns. The selection of the DFECmethod employed may thus be conducted according to an analysis ofcircuit complexity, latency and chip surface area (related to parityoverhead) to optimize the DFEC methods to a particular application. Itshould be noted again, however, that the analysis presented in FIGS.25A-C is provided only as an example of the ability to optimize the DFECmethod employed and the specific configuration thereof to therequirements of a particular application and does not representexperimental data.

Looking now to FIGS. 26A-26E, examples of the five-flag 2-bit ABF methodare illustrated. The following examples examine the use of the five-flagABF method to correct initialization errors in a single subsection of aresistive change element array. As in previous examples, this exemplaryresistive change element array is assumed to have an initializationstate of logic ‘0’, corresponding to the state expected to result in agreater error rate. Further, the following examples demonstrate the useof two-bit subsets and the set of encoding patterns depicted in FIG.21B, containing all possible encoding patterns for a two-bit subset. Asmentioned previously, however, the ABF method is not limited to the useof two-bit subsets, nor to the encoding patterns provided in FIG. 21B.It will be apparent to those skilled in the art that the methods taughtherein may be used to similarly encode subsets of any number of bits,and may employ substantially all or only a selected set of encodingpatterns from the total set of all possible encoding patterns for agiven subset size.

FIG. 26A depicts an encoding operation 2600 a using the five-flag 2-bitABF method of the present disclosure to correct multiple initializationerrors in subsection 2604 a of a resistive change element array. First,subsection 2604 a, comprising a plurality of data cells and five flagcells, is initialized in step 2610 a. Using operations describedpreviously, an initialization operation is performed on all of the datacells within subsection 2604 a to attempt to place them into aninitialized state. Once initialized, subsection 2604 a is read toprovide array data 2614 a, which contains the logical state of each ofthe data cells in the subsection. As can be seen, two subsets 2616 awithin array data 2614 a contain data cells which failed to initializeand remain in an uninitialized state (in this case, a logic ‘1’ state)and may be considered error subsets.

Next, each error subset 2616 a is compared to its corresponding subsetin input data 2612 a to determine the presence of effectiveinitialization errors. That is, the logical state of the error cellswithin subsets 2616 a is compared to the logical state of theircorresponding bits in input data 2612 a. If the logical state of theerror cells match the logical state of the input data, then effectivelyno error has occurred, as the cells are already in the desired state. Inthis case, the input data may not need to be encoded using the ABFmethod. If at least one effective initialization error is detected,however, then the process proceeds to a data pattern matching (DPM)operation, comprising sorting operation 2620 a and matching operation2630 a.

As shown in FIG. 26A, this data pattern matching operation attempts toidentify an encoding pattern which can correct the maximum number ofinitialization errors in subsets 2616 a. To achieve this, the errorsubsets and their corresponding input subsets are selected and sorted instep 2620 a according to input pattern and error pattern, as illustratedwith table 2622 a, wherein each of the initialization errors in subsets2616 a are depicted with an X. Table 2622 a contains two rows todistinguish even (1*) and odd (*1) errors, as well as four columns todistinguish each input pattern (00, 01, 10 and 11). It will be apparentthat a similar table can be constructed for any given subset size. Ascan be seen, the initialization errors in subsets 2616 a comprise oneeven error 10 with a corresponding input pattern of 11, and one odderror 01 with a corresponding input pattern of 00, and have been placedinto table 2622 a accordingly. Next, the error patterns are matched inoperation 2630 a, comprising steps 2632 a, 2634 a, and 2636 a. Matchingoperation 2630 a first selects the input pattern which has the highestnumber of corresponding error pairs (that is, pairs of one even and oneodd error, as will be explained in more detail in FIG. 26B and 26E) totranslate to 11 in step 2632 a. In this case, neither input pattern 00nor input pattern 11 contain an error pair, so the process selects theinput pattern with the largest number of total errors. Since both 00 and11 contain a single error, the process refers to a default selection. Inthis case 00 is selected as default, so 00 is selected to translate to11 and column A is selected from table 2102 of FIG. 21B, whichcorresponds to the set of encoding patterns which translate 00 to 11.

Next, matching operation 2630 a proceeds to step 2634 a, wherein theremaining three input patterns (01, 10 and 11) are compared to selectthe input pattern with the largest number of even errors. Since 11contains one even error, 11 is selected to translate to 10. Thus, rowswithin the column selected in step 2632 a (column A) which translate 11to 10 are selected, corresponding to rows 2 and 6 in table 2102 of FIG.21B. The process then selects the input pattern from the remaining twoinput patterns (01 and 10) which contains the largest number of odderrors. Since neither 01 nor 10 contains an error, the process refers toa default selection. In this case, 01 is selected as default and ischosen to translate as 01. Thus, one of the two rows selected in step2634 a (rows 2 and 6) within the column selected in step 2632 a (columnA) which translate 01 to 01 is selected, corresponding to row 2 of table2102. In this way, the appropriate encoding pattern has been identifiedas the pattern depicted in column A, row 2, of table 2102, referred toas pattern A2. Although not depicted, pattern A2 has a 5-bit patternreference code (PRC) which is stored in the five flag bits in subsection2604 a.

Next, the encoding pattern selected in matching operation 2630 a(pattern A2) is used to encode input data 2612 a in step 2640 a. Thus,all subsets within input data 2612 a which have a pattern 00 aretranslated to 11, all subsets with pattern 11 are translated to 10, allsubsets with pattern 10 are translated to 00, and all subsets withpattern 01 remain unchanged. In this way, input data 2612 a is encodedto produce a set of encoded input data 2642 a. As can be seen, all ofthe error cells in subsets 2616 a are now effectively correct, as theirlogical value matches the value in encoded input data 2642 a.

Finally, encoded input data 2642 a may be further processed by asecondary error correction code, such as BCH, to correct any additionalerrors not corrected in encoding step 2640 a. Encoded input data 2642 ais then programmed into subsection 2604 a in programming step 2650 a. Asdescribed previously, in programming step 2650 a a SET operation isperformed on each bit in the encoded input data requiring a logic ‘1’(since the subsection was initialized to logic ‘0’). Following thisprogramming operation, the data cells in subsection 2604 a have beenprogrammed with encoded input data 2642 a to obtain a programmedsubsection 2654 a, and the five flag cells of subsection 2604 a havebeen programmed with the pattern reference code of encoding patternemployed (pattern A2). As can be seen, at this point all of theinitialization errors in subsets 2616 a have been corrected. Programmedsubsection 2654 a may subsequently be read and decoded by reversing theencoding pattern indicated by the pattern reference code stored in theDFEC flag bits. In this way, the five-flag 2-bit ABF method of thepresent disclosure may be used to correct at least two initializationerrors per subsection.

Similarly, FIG. 26B depicts an encoding operation 2600 b using thefive-flag 2-bit ABF method of the present disclosure to correct multipleinitialization errors in subsection 2604 b of a resistive change elementarray. First, subsection 2604 b, comprising a plurality of data cellsand five flag cells, is initialized in step 2610 b. Using operationsdescribed previously, an initialization operation is performed on all ofthe data cells within subsection 2604 b to attempt to place them into aninitialized state. Once initialized, subsection 2604 b is read toprovide array data 2614 b, which contains the logical state of each ofthe data cells in the subsection. As can be seen, four subsets 2616 bwithin array data 2614 b contain data cells which failed to initializeand remain in an uninitialized state (in this case, a logic ‘1’ state)and may be considered error subsets.

Next, each error subset 2616 b is compared to its corresponding subsetin input data 2612 b to determine the presence of effectiveinitialization errors. That is, the logical state of the error cellswithin subsets 2616 b is compared to the logical state of theircorresponding bits in input data 2612 b. If the logical state of theerror cells match the logical state of the input data, then effectivelyno error has occurred, as the cells are already in the desired state. Inthis case, the input data may not need to be encoded using the ABFmethod. If at least one effective initialization error is detected,however, then the process proceeds to a data pattern matching (DPM)operation, comprising sorting operation 2620 b and matching operation2630 b.

As shown in FIG. 26B, this data pattern matching operation attempts toidentify an encoding pattern which can correct the maximum number ofinitialization errors in subsets 2616 b. To achieve this, the errorsubsets and their corresponding input subsets are selected and sorted instep 2620 b according to input pattern and error pattern, as illustratedwith table 2622 b, wherein each of the initialization errors in subsets2616 b are depicted with an X. Table 2622 b contains two rows todistinguish even (1*) and odd (*1) errors, as well as four columns todistinguish each input pattern (00, 01, 10 and 11). It will be apparentthat a similar table can be constructed for any given subset size. Ascan be seen, the initialization errors in subsets 2616 b comprise twoeven errors 10 with corresponding input patterns of 11 and 10, and twoodd errors 01 with corresponding input patterns of 00 and 10, and havebeen placed into table 2622 b accordingly. Next, the error patterns arematched in operation 2630 b, comprising steps 2632 b, 2634 b, and 2636b. Matching operation 2630 b first selects the input pattern which hasthe highest number of corresponding error pairs to translate to 11 instep 2632 b. In this case, input pattern 10 contains one even and oneodd error, thus it contains one error pair. Since none of the otherinput patterns contain an error pair, input pattern 10 is selected totranslate to 11 and column C is selected from table 2102 of FIG. 21B,which corresponds to the set of encoding patterns which translate 10 to11.

Next, matching operation 2630 b proceeds to step 2634 b, wherein theremaining three input patterns (00, 01 and 11) are compared to selectthe input pattern with the largest number of even errors. Since 11contains one even error, 11 is selected to translate to 10. Thus, rowswithin the column selected in step 2632 b (column C) which translate 11to 10 are selected, corresponding to rows 1 and 3. The process thenselects the input pattern from the remaining two input patterns (00 and01) which contains the largest number of odd errors. Since 00 containsone odd error, 00 is chosen to translate as 01 and one of the two rowsselected in step 2634 b (rows 1 and 3) within the column selected instep 2632 b (column C) which translate 00 to 01 is selected,corresponding to row 3 of table 2102. In this way, the appropriateencoding pattern has been identified as the pattern depicted in columnC, row 3 of table 2102, referred to as pattern C3. Although notdepicted, pattern C3 has a 5-bit pattern reference code (PRC) which isstored in the five flag bits in subsection 2604 b.

Next, the encoding pattern selected in matching operation 2630 b(pattern C3) is used to encode input data 2612 b in step 2640 b. Thus,all subsets within input data 2612 b which have a pattern 00 aretranslated to 01, all subsets with pattern 01 are translated to 00, allsubsets with pattern 10 are translated to 11, and all subsets withpattern 11 are translated to 10. In this way, input data 2612 b isencoded to produce a set of encoded input data 2642 b. As can be seen,all four of the error cells in subsets 2616 b are now effectivelycorrect, as their logical value matches the value in encoded input data2642 b.

Finally, encoded input data 2642 b may be further processed by asecondary error correction code, such as BCH, to correct any additionalerrors not corrected in encoding step 2640 b. Encoded input data 2642 bis then programmed into subsection 2604 b in programming step 2650 b. Asdescribed previously, in programming step 2650 b a SET operation isperformed on each bit in the encoded input data requiring a logic ‘1’(since the subsection was initialized to logic ‘0’). Following thisprogramming operation, the data cells in subsection 2604 b have beenprogrammed with encoded input data 2642 b to obtain a programmedsubsection 2654 b, and the five flag cells of subsection 2604 b havebeen programmed with the pattern reference code of encoding patternemployed (pattern C3). As can be seen, at this point all four of theinitialization errors in subsets 2616 b have been corrected. Programmedsubsection 2654 b may subsequently be read and decoded by reversing theencoding pattern indicated by the pattern reference code stored in theDFEC flag bits. In this way, the five-flag 2-bit ABF method of thepresent disclosure may be used to correct substantially allinitialization errors per subsection, given a certain pattern of errors.

While the error pattern of FIG. 26A and 26B allowed the ABF method tocorrect all of the initialization errors, this is not always the case.As explained previously, the five-flag ABF method guarantees a minimum50% error correction rate in the worst-case distribution of errors. FIG.26C depicts an encoding operation 2600 c using the 5-flag 2-bit ABFmethod of the present disclosure in order to correct multipleinitialization errors which have a worst-case error distribution. First,subsection 2604 c, comprising a plurality of data cells and five flagcells, is initialized in step 2610 c. Using operations describedpreviously, an initialization operation is performed on all of the datacells within subsection 2604 c to attempt to place them into aninitialized state. Once initialized, subsection 2604 c is read toprovide array data 2614 c, which contains the logical state of each ofthe data cells in the subsection. As can be seen, four subsets 2616 cwithin array data 2614 c contain data cells which failed to initializeand remain in an uninitialized state (in this case, a logic ‘1’ state)and may be considered error subsets.

Next, each error subset 2616 c is compared to its corresponding subsetin input data 2612 c to determine the presence of effectiveinitialization errors. That is, the logical state of the error cellswithin subsets 2616 c is compared to the logical state of theircorresponding bits in input data 2612 c. If the logical state of theerror cells match the logical state of the input data, then effectivelyno error has occurred, as the cells are already in the desired state. Inthis case, the input data may not need to be encoded using the ABFmethod. If at least one effective initialization error is detected,however, then the process proceeds to a data pattern matching (DPM)operation, comprising sorting operation 2620 c and matching operation2630 c.

As shown in FIG. 26C, this data pattern matching operation attempts toidentify an encoding pattern which can correct the maximum number ofinitialization errors in subsets 2616 c. To achieve this, the errorsubsets and their corresponding input subsets are selected and sorted instep 2620 c according to input pattern and error pattern, as illustratedwith table 2622 c, wherein each of the initialization errors in subsets2616 c are depicted with an X. As can be seen, the initialization errorsin subsets 2616 c comprise four even errors 10, one corresponding toeach input pattern, and no odd errors. As will be shown, this is aworst-case error distribution, wherein errors of the same type (odd oreven) are distributed evenly across the input patterns. Next, the errorpatterns are matched in operation 2630 c, comprising steps 2632 c, 2634c, and 2636 c. Matching operation 2630 c first selects the input patternwhich has the highest number of corresponding error pairs in step 2632c. In this case, none of the input patterns contain an error pair, sothe process selects the input pattern with the largest number of totalerrors. Since all of the patterns contain a single error, the processrefers to a default selection. In this case 00 is selected as default,so 00 is selected to translate to 11 and column A is selected from table2102 of FIG. 21B.

Next, matching operation 2630 c proceeds to step 2634 c, wherein theremaining three input patterns (01, 10 and 11) are compared to selectthe input pattern with the largest number of even errors. Since allthree input patterns 01, 10 and 11 contain a single even error, theprocess refers to a default selection. In this case 10 is selected asdefault, so 10 is selected to translate to 10. Thus, rows within thecolumn selected in step 2632 c (column A) which translate 10 to 10 areselected, corresponding to rows 1 and 4 in table 2102 of FIG. 21B. Theprocess then selects the input pattern from the remaining two inputpatterns (01 and 11) which contains the largest number of odd errors.Since neither 01 nor 11 contains an odd error, the process refers to adefault selection. In this case, 01 is selected as default and is chosento translate as 01. Thus, one of the two rows selected in step 2634 c(rows 1 and 4) within the column selected in step 2632 c (column A)which translate 01 to 01 is selected, corresponding to row 1 of table2102. In this way, the appropriate encoding pattern has been identifiedas the pattern depicted in column A, row 1, of table 2102, referred toas pattern A1. Although not depicted, pattern A1 has a 5-bit patternreference code (PRC) which is stored in the five flag bits in subsection2604 c.

Next, the encoding pattern selected in matching operation 2630 c(pattern A1) is used to encode input data 2612 c in step 2640 c. Thus,all subsets within input data 2612 c which have a pattern 00 aretranslated to 11, all subsets with pattern 11 are translated to 00, andall subsets with pattern 01 or 10 remain unchanged. In this way, inputdata 2612 c is encoded to produce a set of encoded input data 2642 c. Ascan be seen, two of the error cells in subsets 2616 c are noweffectively correct, as their logical value matches the value in encodedinput data 2642 c, while two error cells remain uncorrected. Thiscorresponds to the worst-case error distribution correction rate of 50%.

Finally, encoded input data 2642 c may be further processed by asecondary error correction code, such as BCH, to correct any additionalerrors not corrected in encoding step 2640 c. This process is necessaryto correct the two initialization errors which were not corrected by theABF method during encoding step 2640 c. Encoded input data 2642 c isthen programmed into subsection 2604 c in programming step 2650 c. Asdescribed previously, in programming step 2650 c a SET operation isperformed on each bit in the encoded input data requiring a logic ‘1’(since the subsection was initialized to logic ‘0’). Following thisprogramming operation, the data cells in subsection 2604 c have beenprogrammed with encoded input data 2642 c to obtain a programmedsubsection 2654 c, and the five flag cells of subsection 2604 c havebeen programmed with the pattern reference code of encoding patternemployed (pattern A1). As can be seen, at this point only half of theinitialization errors in subsets 2616 c have been corrected. Programmedsubsection 2654 c must be read and processed by a secondary errorcorrection method before it can be decoded by the ABF method to retrievethe original input data. In this way, the five-flag 2-bit ABF method ofthe present disclosure may be used to correct at least 50%initialization errors per subsection.

While FIGS. 26A and 26B illustrated best-case error distributions with100% correction rate, and FIG. 26C depicted a worst-case errordistribution with 50% correction rate, most of the possible errordistributions will result in a correction rate between these twoextremes. To this end, FIG. 26D depicts an encoding operation 2600 dusing the five-flag 2-bit ABF method of the present disclosure in orderto correct multiple initialization errors. First, subsection 2604 d,comprising a plurality of data cells and five flag cells, is initializedin step 2610 d. Using operations described previously, an initializationoperation is performed on all of the data cells within subsection 2604 dto attempt to place them into an initialized state. Once initialized,subsection 2604 d is read to provide array data 2614 d, which containsthe logical state of each of the data cells in the subsection. As can beseen, four subsets 2616 d within array data 2614 d contain data cellswhich failed to initialize and remain in an uninitialized state (in thiscase, a logic ‘1’ state) and may be considered error subsets.

Next, each error subset 2616 d is compared to its corresponding subsetin input data 2612 d to determine the presence of effectiveinitialization errors. That is, the logical state of the error cellswithin subsets 2616 d is compared to the logical state of theircorresponding bits in input data 2612 d. If the logical state of theerror cells match the logical state of the input data, then effectivelyno error has occurred, as the cells are already in the desired state. Inthis case, the input data may not need to be encoded using the ABFmethod. If at least one effective initialization error is detected,however, then the process proceeds to a data pattern matching (DPM)operation, comprising sorting operation 2620 d and matching operation2630 d.

As shown in FIG. 26D, this data pattern matching operation attempts toidentify an encoding pattern which can correct the maximum number ofinitialization errors in subsets 2616 d. To achieve this, the errorsubsets and their corresponding input subsets are selected and sorted instep 2620 d according to input pattern and error pattern, as illustratedwith table 2622 d, wherein each of the initialization errors in subsets2616 d are depicted with an X. As can be seen, the initialization errorsin subsets 2616 d comprise three odd errors 01, one corresponding toeach input pattern 00, 01, and 10, and one even error corresponding to11. Next, the error patterns are matched in operation 2630 d, comprisingsteps 2632 d, 2634 d, and 2636 d. Matching operation 2630 d firstselects the input pattern which has the highest number of correspondingerror pairs in step 2632 d. In this case, none of the input patternscontain an error pair, so the process selects the input pattern with thelargest number of total errors. Since all of the patterns contain asingle error, the process refers to a default selection. In this case 00is selected as default, so 00 is selected to translate to 11 and columnA is selected from table 2102 of FIG. 21B.

Next, matching operation 2630 d proceeds to step 2634 d, wherein theremaining three input patterns (01, 10 and 11) are compared to selectthe input pattern with the largest number of even errors. Since only 11contains an even error, 11 is selected to translate to 10, correspondingto rows 2 and 6 in table 2102 of FIG. 21B. The process then selects theinput pattern from the remaining two input patterns (01 and 10) whichcontains the largest number of odd errors. Since both 01 and 10 containan odd error, the process refers to a default selection. In this case,01 is selected as default and is chosen to translate as 01. Thus, one ofthe two rows selected in step 2634 d (rows 2 and 6) within the columnselected in step 2632 d (column A) which translates 01 to 01 isselected, corresponding to row 2 of table 2102. In this way, theappropriate encoding pattern has been identified as the pattern depictedin column A, row 2, of table 2102, referred to as pattern A2. Althoughnot depicted, pattern A2 has a 5-bit pattern reference code (PRC) whichis stored in the five flag bits in subsection 2604 d.

Next, the encoding pattern selected in matching operation 2630 d(pattern A2) is used to encode input data 2612 d in step 2640 d. Thus,all subsets within input data 2612 d which have a pattern 00 aretranslated to 11, all subsets with pattern 11 are translated to 10, allsubsets with pattern 10 are translated to 00, and all subsets withpattern 01 remain unchanged. In this way, input data 2612 d is encodedto produce a set of encoded input data 2642 d. As can be seen, three ofthe error cells in subsets 2616 d are now effectively correct, as theirlogical value matches the value in encoded input data 2642 d, while twoerror cells remain uncorrected. This corresponds to an error correctionrate of 75%, which is less than the maximum but higher than the minimumcorrection rate.

Finally, encoded input data 2642 d may be further processed by asecondary error correction code, such as BCH, to correct any additionalerrors not corrected in encoding step 2640 d. This process is necessaryto correct the remaining initialization error which was not corrected bythe ABF method during encoding step 2640 d. Encoded input data 2642 d isthen programmed into subsection 2604 d in programming step 2650 d. Asdescribed previously, in programming step 2650 d a SET operation isperformed on each bit in the encoded input data requiring a logic ‘1’(since the subsection was initialized to logic ‘0’). Following thisprogramming operation, the data cells in subsection 2604 d have beenprogrammed with encoded input data 2642 d to obtain a programmedsubsection 2654 d, and the five flag cells of subsection 2604 d havebeen programmed with the pattern reference code of encoding patternemployed (pattern A2). As can be seen, at this point only three of theinitialization errors in subsets 2616 d have been corrected. Programmedsubsection 2654 d must be read and processed by a secondary errorcorrection method before it can be decoded to recover the original inputdata. In this way, the five-flag 2-bit ABF method of the presentdisclosure may be used to correct between 100% and 50% of initializationerrors in a subsection, dependent on the error distribution.

FIG. 26E provides a further demonstration of the data pattern matchingprocess to determine the optimal encoding pattern to address a givenerror distribution, as well as the handling of continuous errors. FIG.26E depicts an encoding operation 2600 e using the five-flag 2-bit ABFmethod of the present disclosure in order to correct multipleinitialization errors. First, subsection 2604 e, comprising a pluralityof data cells and five flag cells, is initialized in step 2610 e. Usingoperations described previously, an initialization operation isperformed on all of the data cells within subsection 2604 e to attemptto place them into an initialized state. Once initialized, subsection2604 e is read to provide array data 2614 e, which contains the logicalstate of each of the data cells in the subsection. As can be seen, fivesubsets 2616 e within array data 2614 e contain data cells which failedto initialize and remain in an uninitialized state (in this case, alogic ‘1’ state) and may be considered error subsets.

Next, each error subset 2616 e is compared to its corresponding subsetin input data 2612 e to determine the presence of effectiveinitialization errors. That is, the logical state of the error cellswithin subsets 2616 e is compared to the logical state of theircorresponding bits in input data 2612 e. If the logical state of theerror cells match the logical state of the input data, then effectivelyno error has occurred, as the cells are already in the desired state. Inthis case, the input data may not need to be encoded using the ABFmethod. If at least one effective initialization error is detected,however, then the process proceeds to a data pattern matching (DPM)operation, comprising sorting operation 2620 e and matching operation2630 e.

As shown in FIG. 26E, this data pattern matching operation attempts toidentify an encoding pattern which can correct the maximum number ofinitialization errors in subsets 2616 e. To achieve this, the errorsubsets and their corresponding input subsets are selected and sorted instep 2620 e according to input pattern and error pattern, as illustratedwith table 2622 e, wherein each of the initialization errors in subsets2616 e are depicted with an X. As can be seen, the initialization errorsin subsets 2616 e comprise one even and one odd error corresponding toinput pattern 11, two even errors 10 corresponding to input pattern 00,and one even error corresponding to 10. In some applications, such acontinuous error as the one which has occurred with input pattern 11 maybe treated as one even and one odd error, even though they have occurredin the same subset. Next, the error patterns are matched in operation2630 e, comprising steps 2632 e, 2634 e, and 2636 e. Matching operation2630 e first selects the input pattern which has the highest number ofcorresponding error pairs in step 2632 e. In this case, one error pairhas occurred for input pattern 11, so 11 is selected to translate to 11and column D is selected from table 2102 of FIG. 21B.

Next, matching operation 2630 e proceeds to step 2634 e, wherein theremaining three input patterns (00, 01 and 10) are compared to selectthe input pattern with the largest number of even errors. Since 00contains two even errors, 00 is selected to translate to 10,corresponding to rows 4 and 6 in table 2102 of FIG. 21B. The processthen selects the input pattern from the remaining two input patterns (01and 10) which contains the largest number of odd errors. Since neither01 nor 10 contain an odd error, the process refers to a defaultselection. In this case, 01 is selected as default and is chosen totranslate as 01. Thus, one of the two rows selected in step 2634 e (rows4 and 6) within the column selected in step 2632 e (column D) whichtranslates 01 to 01 is selected, corresponding to row 4 of table 2102.In this way, the appropriate encoding pattern has been identified as thepattern depicted in column D, row 4, of table 2102, referred to aspattern D4. Although not depicted, pattern D4 has a 5-bit patternreference code (PRC) which is stored in the five flag bits in subsection2604 e.

Next, the encoding pattern selected in matching operation 2630 e(pattern D4) is used to encode input data 2612 e in step 2640 e. Thus,all subsets within input data 2612 e which have a pattern 00 aretranslated to 10, all subsets with pattern 10 are translated to 00, andall subsets with pattern 01 or 11 remain unchanged. In this way, inputdata 2612 e is encoded to produce a set of encoded input data 2642 e. Ascan be seen, four of the error cells in subsets 2616 e are noweffectively correct, as their logical value matches the value in encodedinput data 2642 e, while one error cell remains uncorrected. Thiscorresponds to an error correction rate of 80%, which is less than themaximum but higher than the minimum correction rate.

Finally, encoded input data 2642 e may be further processed by asecondary error correction code, such as BCH, to correct any additionalerrors not corrected in encoding step 2640 e. This process is necessaryto correct the remaining initialization error which was not corrected bythe ABF method during encoding step 2640 e. Encoded input data 2642 e isthen programmed into subsection 2604 e in programming step 2650 e. Asdescribed previously, in programming step 2650 e a SET operation isperformed on each bit in the encoded input data requiring a logic ‘1’(since the subsection was initialized to logic ‘0’). Following thisprogramming operation, the data cells in subsection 2604 e have beenprogrammed with encoded input data 2642 e to obtain a programmedsubsection 2654 e, and the five flag cells of subsection 2604 e havebeen programmed with the pattern reference code of encoding patternemployed (pattern D4). As can be seen, at this point only four of theinitialization errors in subsets 2616 e have been corrected. Programmedsubsection 2654 e must be read and processed by a secondary errorcorrection method before it can be decoded to recover the original inputdata. In this way, the 5-flag 2-bit ABF method of the present disclosuremay be used to correct between 100% and 50% of initialization errors ina subsection, dependent on the error distribution.

Finally, FIG. 26F provides a demonstration of the data pattern matchingprocess to determine the optimal encoding pattern with an additionalselection criteria. FIG. 26F depicts an encoding operation 2600 f usingthe five-flag 2-bit ABF method of the present disclosure in order tocorrect multiple initialization errors. First, subsection 2604 f,comprising a plurality of data cells and five flag cells, is initializedin step 2610 f. Using operations described previously, an initializationoperation is performed on all of the data cells within subsection 2604 fto attempt to place them into an initialized state. Once initialized,subsection 2604 f is read to provide array data 2614 f, which containsthe logical state of each of the data cells in the subsection. As can beseen, four subsets 2616 f within array data 2614 f contain data cellswhich failed to initialize and remain in an uninitialized state (in thiscase, a logic ‘1’ state) and may be considered error subsets.

Next, each error subset 2616 f is compared to its corresponding subsetin input data 2612 f to determine the presence of effectiveinitialization errors. That is, the logical state of the error cellswithin subsets 2616 f is compared to the logical state of theircorresponding bits in input data 2612 f. If the logical state of theerror cells match the logical state of the input data, then effectivelyno error has occurred, as the cells are already in the desired state. Inthis case, the input data may not need to be encoded using the ABFmethod. If at least one effective initialization error is detected,however, then the process proceeds to a data pattern matching (DPM)operation, comprising sorting operation 2620 f and matching operation2630 f.

As shown in FIG. 26F, this data pattern matching operation attempts toidentify an encoding pattern which can correct the maximum number ofinitialization errors in subsets 2616 f. To achieve this, the errorsubsets and their corresponding input subsets are selected and sorted instep 2620 f according to input pattern and error pattern, as illustratedwith table 2622 f, wherein each of the initialization errors in subsets2616 f are depicted with an X. As can be seen, the initialization errorsin subsets 2616 f comprise one odd error 01 corresponding to inputpattern 11, two odd errors 01 corresponding to input pattern 01, and oneeven error corresponding to 10. Next, the error patterns are matched inoperation 2630 f, comprising steps 2632 f, 2634 f, and 2636 f, inaddition to step 2633 f. Matching operation 2630 f first selects theinput pattern which has the highest number of corresponding error pairsin step 2632 f. As can be seen, no error pairs have occurred and theprocess then selects the input pattern with the highest number of totalerrors to translate to 11 in step 2633 f. Input pattern 01 has twoerrors and is thus selected to translate to 11, corresponding to columnB of FIG. 21B. Step 2633 f is only performed if no error pairs aredetected in step 2632 f, and, as demonstrated previously, if multipleinput patterns have the same number of errors (if both 00 and 10 havetwo errors, for instance) then the process may refer to a defaultsetting.

Next, matching operation 2630 f proceeds to step 2634 f, wherein theremaining three input patterns (00, 10 and 11) are compared to selectthe input pattern with the largest number of even errors. Since 10contains one even error, 10 is selected to translate to 10,corresponding to rows 1 and 4 in table 2102 of FIG. 21B. The processthen selects the input pattern from the remaining two input patterns (00and 11) which contains the largest number of odd errors. Since input 11has one odd error, 11 is selected to translate to 01. Thus, one of thetwo rows selected in step 2634 f (rows 1 and 4) within the columnselected in step 2632 f (column B) which translates 11 to 01 isselected, corresponding to row 1 of table 2102. In this way, theappropriate encoding pattern has been identified as the pattern depictedin column B, row 1, of table 2102, referred to as pattern B1. Althoughnot depicted, pattern B1 has a 5-bit pattern reference code (PRC) whichis stored in the five flag bits in subsection 2604 f.

Next, the encoding pattern selected in matching operation 2630 f(pattern B1) is used to encode input data 2612 f in step 2640 f. Thus,all subsets within input data 2612 f which have a pattern 01 aretranslated to 11, all subsets with pattern 11 are translated to 01, andall subsets with pattern 00 or 10 remain unchanged. In this way, inputdata 2612 f is encoded to produce a set of encoded input data 2642 f. Ascan be seen, all four of the error cells in subsets 2616 f are noweffectively correct, as their logical value matches the value in encodedinput data 2642 f. This corresponds to an error correction rate of 100%,which is only possible because of additional matching step 2633 f.

Finally, encoded input data 2642 f may be further processed by asecondary error correction code, such as BCH, to correct any additionalerrors not corrected in encoding step 2640 f. This process is necessaryto correct the remaining initialization error which was not corrected bythe ABF method during encoding step 2640 f. Encoded input data 2642 f isthen programmed into subsection 2604 f in programming step 2650 f. Asdescribed previously, in programming step 2650 f a SET operation isperformed on each bit in the encoded input data requiring a logic ‘1’(since the subsection was initialized to logic ‘0’). Following thisprogramming operation, the data cells in subsection 2604 f have beenprogrammed with encoded input data 2642 f to obtain a programmedsubsection 2654 f, and the five flag cells of subsection 2604 f havebeen programmed with the pattern reference code of encoding patternemployed (pattern B1). As can be seen, at this point all four of theinitialization errors in subsets 2616 f have been corrected, andprogrammed subsection 2654 f may be read and decoded by reversing theencoding pattern to recover the input data. In this way, the five-flag2-bit ABF method of the present disclosure may be used to correct themaximum number of initialization errors in a subsection.

DFEC Hardware Systems

Referring now to FIG. 27, a system level block diagram is shownillustrating a memory system 2700 comprising a resistive change elementarray 2740 and suitable for use with the DFEC methods of the presentdisclosure.

At the core of the access and addressing system 2700 is a resistivechange element array 2740 similar in architecture to the arrays shown inFIGS. 2, 4, and 5. A processor control element 2710 provides an array ofaddress control lines to a bit line driver/buffer circuit 2720 and to aword line driver/buffer circuit 2730. The bit line driver/buffer circuit2720 then generates an array of bit lines through bit line decoderelement 2725 and provides those bit lines to resistive change elementarray 2740. Similarly, the word line driver/buffer circuit 2730generates an array of word lines through word line decoder element 2735and provides those word lines to resistive change element array 2740. Itshould be noted that order of the driver, buffer and decoder circuitsfor the bit line and word line may vary depending on the needs of aparticular application. Thus, the buffer may be placed first, followedby the decoder, then the driver, or in any other order, the invention isnot limited in this regard. Further, in some applications the driver maybe unnecessary and may be removed. In this way, electrical stimuli canbe provided to the resistive change element array 2740 from processorcontrol element 2710 in order to adjust the state of cells within thearray (as described with respect to FIGS. 2, 3, 4, and 5 above).

A DFEC encoder circuit 2752 (such as exemplary encoder circuit 1201 inFIG. 12A or 2001 in FIG. 20A) is responsive to resistive change elementarray 2740 and provides programming data (analogous to 1240 in FIG. 12Aor 2050 in FIG. 20A) to a BCH error correction circuit 2754. The BCHerror correction circuit 2754 then provides BCH encoded data back toresistive change element array 2740. The resistive change element array2740 is then able to output read data first through BCH ECC decoder 2762and then through DFEC decoder circuit 2764 (such as exemplary decodercircuit 1202 in FIG. 12B or 2002 in FIG. 20B). In this way, memorysystem 2700 is able to execute the DFEC methods of the presentdisclosure as detailed in FIGS. 10, 18 and 23, and described throughoutthe present specification.

DFEC decoder circuit 2764 provides corrected data to an array of senseamplifiers 2774 through an analog multiplexer element 2772. Responsiveto control signals from the processor control element 2710, the analogmultiplexer element 2772 interconnects the bit lines, words lines, and,in some cases, reference bit lines (as shown in FIG. 2, for example) tothe array of sense amplifiers 2774. A system of I/O gates 2776 isresponsive to the array of sense amplifiers 2774 and control signalsfrom the processor control element 2710 and is used to temporally latchand store logic values read from the resistive change element array.Responsive to the I/O gate element 2776, a data buffer driver element2780 provides the corrected data values read from the array back to theprocessor control element 2710.

The processor control element 2710 within the exemplary access andaddressing system of FIG. 27 is used to represent a programmingoperation circuit (or the like) that can be used to apply the differentvoltages and other conditions to the arrays of bit lines and word lineswithin a resistive change element array as required by the methods ofthe present disclosure and discussed with respect to the figures above.Such electrical stimuli can be implemented through a variety ofstructures as best fits the needs of a specific application. Forexample, FPGAs, PLDs, microcontrollers, logic circuits, or a softwareprogram executing on a computer could all be used to execute the dynamicprogramming operations and dynamic READ operations as detailed in theprevious discussions.

It should be noted that though the resistive change memory arrayarchitectures of the present disclosure are presented using theexemplary simplified schematics within FIGS. 2, 4, 5, 8B, 9B, 12A-12B,and 20A-20B, and the block diagrams of FIGS. 8A, 9A, 17A, 22A and 25,the methods of the present disclosure should not be limited to thosespecific electrical circuits depicted. Indeed, it will be clear to thoseskilled in the art that the electrical circuits depicted in thesefigures can be altered in a plurality of ways to optimize a circuit topractice the described error correction methods within a specificapplication.

It is preferred, then, that the preceding description of resistivechange memory array architectures be representative and inclusive ofthese variations and not otherwise limited to the specific illustrativeparameters detailed.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention not be limited by thespecific disclosure herein.

What is claimed is:
 1. A method of error correction for a resistivechange element array comprising: providing a resistive change elementarray, said resistive change element array comprising: a plurality ofresistive change elements, each of said plurality of resistive changeelements capable of adjusting between at least two non-volatileresistive states responsive to applied electrical stimuli, wherein oneof said non-volatile resistive states is an initialized state; whereinsaid plurality of resistive change elements within said array areorganized into a plurality of subsections, each of said subsectionsincluding at least one subset, each of said at least one subsetsincluding a plurality of resistive change elements assigned as datacells and one resistive change element assigned as a flag cell;receiving a set of input data to be programmed into at least one of saidsubsections; initializing all of said data cells and said flag cellswithin at least one of said subsections into said initialized state;reading said data cells within said at least one initialized subsectionto identify any data cells with an initialization error; comparing saiddata cells with an initialization error to corresponding bits withinsaid input data to identify effective initialization errors, whereinsaid effective initialization errors are said data cells with aninitialization error which have a different logical value to saidcorresponding bits within said set of input data; activating said flagcell of each of said subsets within said initialized subsection whichcontain at least one effective initialization error; encoding said inputdata by inverting all input data bits corresponding to said data cellswithin each of said subsets containing an activated flag cell togenerate encoded input data; programming said encoded input data into atleast one of said initialized subsections; accessing said programmeddata during a read operation to obtain encoded read data; and decodingsaid encoded read data by inverting all read data bits corresponding tosaid data cells within each subset containing an activated flag cell togenerate output data; wherein each of said subsets comprises a number ofdata cells equal to the number of data cells in said subsection dividedby the number of flag cells in said subsection; and wherein saidencoding step provides a first error correction operation.
 2. The methodof claim 1 wherein said resistive change element array further compriseserror correction parity cells, said encoded input data is processedthrough a second error correction operation prior to said step ofprogramming, and said step of programming further comprises programmingsaid parity cells with a parity code according to said second errorcorrection operation.
 3. The method of claim 2 wherein said encoded readdata is processed through said second error correction operation andcorrected responsive to said parity code and said second errorcorrection operation prior to said step of decoding.
 4. The method ofclaim 3 wherein said input data and said encoded input data remainsunchanged by said second error correction operation during saidprocessing steps.
 5. The method of claim 2 wherein the number of saidflag cells within said resistive change element array and the number ofparity cells within said resistive change element array define acombined parity overhead, a combined correctable bit error rate (BER),and a combined error correction latency for said resistive changeelement array.
 6. The method of claim 5 wherein at least one of saidcombined parity overhead, said combined correctable bit error rate(BER), said combined error correction latency is improved as compared toa respective equivalent parity overhead, equivalent combined correctablebit error rate (BER), and equivalent error correction latency that wouldbe realized using said second error correction operation alone.
 7. Themethod of claim 2 wherein said second error correction operationcorrects programming errors, data retention errors, and initializationerrors not corrected by said first error correction operation.
 8. Themethod of claim 1 wherein said array possesses a bias towards errorduring programming said data cells into one of said non-volatileresistive states.
 9. The method of claim 8 wherein said biasednon-volatile resistive state is selected as said initialized state. 10.The method of claim 1 wherein said first error correction operation iscapable of correcting at least one effective initialization error persubset within said array.
 11. The method of claim 1 wherein said firsterror correction operation provides a preselected correctable bit errorrate (BER).
 12. The method of claim 11 wherein said preselectedcorrectable bit error rate (BER) is within the range of 0.1% to 0.2%.13. The method of claim 11 wherein said preselected correctable biterror rate (BER) is within the range of 0.2% to 0.5%.
 14. The method ofclaim 1 wherein each of said subsections comprises a single subset. 15.The method of claim 1 wherein each of said subsections comprise twosubsets.
 16. The method of claim 15 wherein one of said subsetscomprises even data cells and the other comprises odd data cells. 17.The method of claim 1 where said steps of comparing, encoding anddecoding are performed using an electrical circuit.
 18. The method ofclaim 1 wherein said steps of encoding and decoding are performed usinga software algorithm.
 19. The method of claim 18 wherein said softwarealgorithm is executed within one of a microprocessor, a microcontroller,an FPGA, a PLD, and a computer.
 20. The method of claim 1 wherein saidresistive change elements are two-terminal nanotube switching elements.